Lysosomal storage diseases: follow the stored material

1. A. Idursulfase (Why this does not fit)

   Idursulfase replaces iduronate-2-sulfatase in MPS II, not acid β-glucosidase in Gaucher disease.

   Reasoning steps for option A

   1. Why might idursulfase come up when a child has organomegaly and bone pain?

      Idursulfase is a well-known lysosomal enzyme replacement, and MPS II also causes hepatosplenomegaly and skeletal disease.

   2. Which laboratory result rules idursulfase out?

      The measured defect is acid β-glucosidase, not iduronate-2-sulfatase, so replacing IDS would not treat this child.

2. B. Laronidase (Why this does not fit)

   Laronidase replaces α-L-iduronidase in MPS I and does not correct the GBA1 enzyme defect described here.

   Reasoning steps for option B

   1. What makes laronidase look relevant for this skeletal and visceral picture?

      Laronidase treats MPS I, which also combines organomegaly with bone and joint disease.

   2. Which enzyme would laronidase need to be missing, and is it missing here?

      Laronidase supplies α-L-iduronidase; this child instead lacks acid β-glucosidase, so the replacement does not match the defect.

3. C. Imiglucerase (Best answer)

   The visceral, hematologic, and skeletal pattern with low acid β-glucosidase identifies Gaucher disease; imiglucerase is enzyme replacement for systemic Gaucher manifestations.

   Reasoning steps for option C

   1. Which features identify Gaucher disease in this 3-year-old?

      Marked splenomegaly, anemia, thrombocytopenia, bone pain, and distal femoral flaring are the macrophage-driven Gaucher pattern, and acid β-glucosidase is 5% of reference.

   2. Why is imiglucerase the enzyme-matched therapy?

      Imiglucerase is recombinant acid β-glucosidase, so it replaces exactly the enzyme this child lacks and improves systemic Gaucher manifestations.

4. D. Alglucosidase alfa (Why this does not fit)

   Alglucosidase alfa replaces acid α-glucosidase in Pompe disease, whose major targets are cardiac and skeletal muscle glycogen storage.

   Reasoning steps for option D

   1. Why could alglucosidase alfa seem appropriate for a child with a lysosomal enzyme deficiency?

      It is another intravenous lysosomal enzyme replacement, and its name resembles the glucosidase deficiency reported here.

   2. What is the key difference between the enzyme it replaces and the one that is low here?

      Alglucosidase alfa replaces acid α-glucosidase for Pompe muscle glycogen storage, whereas this child lacks acid β-glucosidase with macrophage storage.

Takeaway: Symptomatic systemic Gaucher disease can be treated with acid β-glucosidase enzyme replacement; the treatment must match the deficient enzyme.

Case sources: ^[3]

1. A. Each son has a 50% chance of inheriting the variant from him (Why this does not fit)

   A father gives his Y chromosome, not his X chromosome, to sons, so an affected man does not transmit his GLA variant directly to sons.

   Reasoning steps for option A

   1. Why might a father seem to pass an X-linked variant to half his sons?

      The 50% rule is familiar from heterozygous mothers and is easily misapplied to affected fathers.

   2. Which sex chromosome does this man actually give to each son?

      He gives every son his Y chromosome, so no son can receive his X-linked GLA variant from him.

2. B. All daughters inherit his GLA variant, while none of his sons inherit it from him (Best answer)

   GLA is on the X chromosome. An affected man transmits his only X chromosome to every daughter and his Y chromosome to every son.

   Reasoning steps for option B

   1. Which findings confirm that this man has X-linked Fabry disease?

      Acroparesthesias, angiokeratomas, cornea verticillata, albuminuria, an affected maternal uncle, and a pathogenic GLA variant fit Fabry disease.

   2. How does his single X chromosome determine transmission to his children?

      Every daughter receives his only X chromosome and therefore the GLA variant, while every son receives his Y chromosome instead.

3. C. Only daughters who also inherit a second GLA variant can have clinical disease (Why this does not fit)

   Fabry disease is X-linked, and heterozygous females can be clinically affected because X-chromosome expression is variable; a second pathogenic GLA variant is not required.

   Reasoning steps for option C

   1. Why might someone think daughters need two GLA variants to be affected?

      Recessive thinking suggests that one normal copy should protect a female, as in autosomal recessive disease.

   2. What does Fabry biology show about heterozygous females?

      Variable X inactivation lets heterozygous females develop renal, cardiac, or neurologic Fabry disease with a single GLA variant.

4. D. The variant is transmitted equally to sons and daughters because Fabry disease is autosomal recessive (Why this does not fit)

   The maternal male pattern and confirmed GLA disorder fit X-linked inheritance, not autosomal recessive transmission.

   Reasoning steps for option D

   1. Why could autosomal recessive inheritance seem plausible for a lysosomal storage disease?

      Most lysosomal storage diseases in this lesson are autosomal recessive.

   2. Which family and molecular clues point away from autosomal recessive inheritance?

      The affected maternal uncle and the GLA gene on the X chromosome indicate X-linked inheritance, not equal transmission to sons and daughters.

Takeaway: Fabry disease is X-linked: affected fathers transmit the GLA variant to all daughters and no sons, and heterozygous females can have clinically important disease.

Case sources: ^[4]

1. A. Lysosomal degradation of glycogen in cardiac and skeletal muscle (Best answer)

   Infantile Pompe disease is caused by acid α-glucosidase deficiency, so glycogen accumulates within lysosomes of cardiac and skeletal muscle and produces cardiomyopathy and hypotonia.

   Reasoning steps for option A

   1. Which findings in this infant localize disease to cardiac and skeletal muscle?

      Severe hypotonia, macroglossia, hypertrophic cardiomyopathy, and a short PR with large QRS complexes point to infantile-onset Pompe disease.

   2. Where does glycogen accumulate when acid α-glucosidase is deficient?

      Acid α-glucosidase degrades glycogen inside lysosomes, so its 2% activity causes lysosomal glycogen storage in heart and skeletal muscle.

2. B. Cytosolic cleavage of α-1,6 glycogen branch points in hepatocytes (Why this does not fit)

   A cytosolic glycogen branch-point defect causes a different glycogenosis and does not explain the measured lysosomal acid α-glucosidase deficiency.

   Reasoning steps for option B

   1. Why might a glycogen debranching defect be considered in an infant with muscle and heart disease?

      Debrancher deficiency is a glycogen storage disease and can involve both liver and muscle.

   2. Which measured result separates this from a cytosolic debranching defect?

      The low enzyme is lysosomal acid α-glucosidase, and the dominant phenotype is cardiomyopathy rather than a hepatocyte branch-point block.

3. C. Lysosomal degradation of GM2 ganglioside in retinal ganglion cells (Why this does not fit)

   GM2 degradation is impaired in Tay-Sachs or Sandhoff disease, which produces neurodegeneration rather than the dominant cardiomyopathy and myopathy here.

   Reasoning steps for option C

   1. Why could GM2 degradation seem relevant in a hypotonic 5-month-old?

      GM2 gangliosidoses cause infantile hypotonia and are also lysosomal degradation defects.

   2. What is the stored substrate here, and does it match GM2 disease?

      The stored substrate is glycogen, because acid α-glucosidase is deficient, so a GM2 ganglioside defect does not explain the cardiomyopathy.

4. D. Lysosomal degradation of heparan and dermatan sulfate in connective tissues (Why this does not fit)

   That pattern belongs to MPS I or II and would not explain the very low acid α-glucosidase activity or classic infantile cardiac-muscle phenotype.

   Reasoning steps for option D

   1. Why could a glycosaminoglycan defect be considered in an infant with macroglossia?

      Mucopolysaccharidoses can cause an enlarged tongue, and they are also lysosomal degradation disorders.

   2. Which enzyme result makes a GAG degradation defect the wrong mechanism?

      The deficient enzyme degrades glycogen, not dermatan or heparan sulfate, and the dominant hypertrophic cardiomyopathy fits Pompe disease.

Takeaway: Pompe disease is a lysosomal glycogenosis: GAA deficiency causes glycogen storage in muscle lysosomes, producing infantile cardiomyopathy and hypotonia or later myopathy.

Case sources: ^[8]

1. A. Macrophages with crumpled-tissue-paper cytoplasm in bone marrow (Why this does not fit)

   Those are Gaucher cells associated with GBA1 deficiency and systemic macrophage storage, not a GALC-deficient demyelinating leukodystrophy.

   Reasoning steps for option A

   1. Why could a macrophage storage finding seem relevant in an infant with neurodegeneration?

      Type 2 Gaucher disease causes infantile neurologic decline, and Gaucher cells are a classic lysosomal pathology finding.

   2. Which result excludes Gaucher cells as the supporting pathology?

      Gaucher disease requires acid β-glucosidase deficiency, whereas this infant has absent galactocerebrosidase with central and peripheral demyelination.

2. B. Metachromatic sulfatide deposits with arylsulfatase A deficiency (Why this does not fit)

   Metachromatic leukodystrophy can cause central and peripheral demyelination, but its biochemical defect is ARSA deficiency with sulfatide accumulation, not absent galactocerebrosidase.

   Reasoning steps for option B

   1. Why is metachromatic leukodystrophy a close competitor in this infant?

      MLD is also a lysosomal leukodystrophy with white-matter disease and slowed peripheral nerve conduction.

   2. Which laboratory result points away from MLD?

      The undetectable galactocerebrosidase activity identifies Krabbe disease, whereas MLD requires arylsulfatase A deficiency with sulfatide excess.

3. C. Multinucleated globoid macrophages in affected white matter (Best answer)

   GALC deficiency with infantile central and peripheral demyelination identifies Krabbe disease, whose characteristic pathology includes globoid macrophages in white matter.

   Reasoning steps for option C

   1. Which findings identify infantile Krabbe disease?

      Irritability, feeding difficulty, hypertonia, slowed nerve conduction, diffuse white-matter abnormality, and undetectable galactocerebrosidase fit infantile Krabbe disease.

   2. What pathologic finding accompanies GALC deficiency in white matter?

      Galactosylceramide-laden multinucleated globoid macrophages collect in demyelinating white matter and give the disease its alternate name.

4. D. Foam cells from acid sphingomyelinase deficiency in liver and spleen (Why this does not fit)

   ASMD produces sphingomyelin-laden visceral macrophages and often organomegaly; it does not fit the absent GALC activity and peripheral neuropathy pattern.

   Reasoning steps for option D

   1. Why might foam cells be considered in an infant with rapid neurologic decline?

      Infantile neurovisceral ASMD also causes early neurodegeneration and has characteristic lipid-laden foam cells.

   2. Which findings make ASMD foam cells the wrong supporting pathology?

      No organomegaly is described, and the absent galactocerebrosidase with peripheral demyelination points to Krabbe disease rather than acid sphingomyelinase deficiency.

Takeaway: Krabbe disease combines GALC deficiency, central and peripheral demyelination, and globoid macrophages; the enzyme result separates it from MLD.

Case sources: ^{[7] [13]}

1. A. Deficient acid sphingomyelinase activity from SMPD1 variants (Why this does not fit)

   ASMD can cause visceral and neurologic disease, but the adolescent vertical saccadic palsy and abnormal intracellular cholesterol trafficking assay point to NPC rather than sphingomyelinase deficiency.

   Reasoning steps for option A

   1. Why does ASMD enter the differential for this adolescent?

      The historical Niemann-Pick name links ASMD with NPC, and ASMD can combine visceral and neurologic disease.

   2. Which two findings favor NPC over acid sphingomyelinase deficiency?

      Vertical supranuclear saccadic palsy and abnormal filipin staining of unesterified cholesterol indicate an NPC trafficking defect.

2. B. Impaired NPC1 or NPC2 dependent intracellular lipid trafficking (Best answer)

   The combination of earlier visceral disease, adolescent neurologic decline, vertical supranuclear saccadic palsy, and abnormal cholesterol handling is characteristic of NPC1/NPC2 dysfunction.

   Reasoning steps for option B

   1. Which clinical sequence in this boy is characteristic of NPC?

      Neonatal cholestasis followed years later by ataxia, dysarthria, school decline, and impaired vertical saccades is a classic NPC course.

   2. What does filipin staining add to the clinical picture?

      It shows unesterified cholesterol trapped in late endosomes and lysosomes, which reflects impaired NPC1 or NPC2 lipid trafficking.

3. C. Deficient hexosaminidase A activity with neuronal GM2 accumulation (Why this does not fit)

   HEXA deficiency can cause later-onset neurologic disease, but it does not explain the classic cholesterol-trafficking assay or neonatal cholestatic history.

   Reasoning steps for option C

   1. Why could a later-onset HEXA disorder be considered here?

      Juvenile and later-onset GM2 disease cause ataxia, dysarthria, and cognitive decline in adolescents.

   2. Which history and test result does a HEXA disorder fail to explain?

      HEXA disorders do not cause neonatal cholestasis or abnormal filipin cholesterol staining.

4. D. Deficient lysosomal acid lipase with cholesteryl ester accumulation (Why this does not fit)

   LAL-D produces hepatic disease and dyslipidemia, but the vertical saccadic palsy and filipin pattern are much more consistent with NPC.

   Reasoning steps for option D

   1. Why might lysosomal acid lipase deficiency seem relevant?

      LAL-D is also a lysosomal lipid disorder with liver involvement and cholesterol-related findings.

   2. Which findings point away from LAL-D?

      LAL-D lacks progressive ataxia and vertical saccadic palsy, and its defect is cholesteryl ester hydrolysis rather than trafficking of unesterified cholesterol.

Takeaway: NPC is a lipid-trafficking disorder, not an acid sphingomyelinase deficiency; vertical supranuclear saccadic palsy plus abnormal cellular cholesterol handling is a strong combination.

Case sources: ^[6]

1. A. An affected father transmits the disorder to all sons because the gene is on the Y chromosome (Why this does not fit)

   MPS II is caused by IDS variants on the X chromosome, so there is no father-to-son transmission through a Y-linked mechanism.

   Reasoning steps for option A

   1. Why might someone think a disease that affects only boys is Y-linked?

      A male-only pattern can suggest a gene on the Y chromosome.

   2. Where is the IDS gene, and what does that mean for fathers and sons?

      IDS is on the X chromosome, so an affected father passes his Y chromosome, not the variant, to sons.

2. B. The disorder is autosomal recessive, so each sibling has a 25% risk regardless of sex (Why this does not fit)

   That recurrence rule applies to autosomal recessive disorders such as MPS I, not IDS deficiency.

   Reasoning steps for option B

   1. Why could autosomal recessive counseling seem appropriate for this MPS picture?

      Most mucopolysaccharidoses, including MPS I, are autosomal recessive with a 25% sibling risk.

   2. Which enzyme result makes the 25% rule wrong for this family?

      Iduronate-2-sulfatase deficiency is MPS II, which is X-linked, so risk depends on the sex of the child and the mother's carrier status.

3. C. Females cannot carry or manifest the disorder because they have two X chromosomes (Why this does not fit)

   Heterozygous females can carry an IDS variant and rare females can manifest MPS II, especially when X-chromosome biology favors expression of the pathogenic allele.

   Reasoning steps for option C

   1. Why might females seem unable to be affected by an X-linked disorder?

      A second X chromosome usually supplies a normal allele, so heterozygous females are often called unaffected carriers.

   2. What can happen in a heterozygous female with an IDS variant?

      She can carry and transmit the variant, and skewed X inactivation can occasionally cause clinical MPS II.

4. D. A heterozygous mother can transmit the IDS variant to half of sons and half of daughters (Best answer)

   MPS II is X-linked. A heterozygous mother has a 50% chance to transmit the affected X chromosome in each pregnancy, with sons who inherit it generally affected and daughters who inherit it heterozygous.

   Reasoning steps for option D

   1. Which findings in this boy identify MPS II?

      Coarse features, joint stiffness, hearing loss, hepatosplenomegaly, clear corneas, and iduronate-2-sulfatase at 3% of reference fit MPS II.

   2. How does a heterozygous mother transmit an X-linked IDS variant?

      Each child has a 50% chance of receiving her variant X; sons who inherit it are usually affected and daughters who inherit it are heterozygous.

Takeaway: MPS II is X-linked IDS deficiency: there is no father-to-son transmission, and heterozygous females are not biologically irrelevant.

Case sources: ^[10]

1. A. Sandhoff disease (Best answer)

   Combined loss of hexosaminidase A and B identifies HEXB deficiency, and the organomegaly supports Sandhoff over classic Tay-Sachs.

   Reasoning steps for option A

   1. Which enzyme pattern identifies Sandhoff disease here?

      Undetectable hexosaminidase A and B together indicate loss of the shared β subunit encoded by HEXB.

   2. How does the organ exam support Sandhoff over classic Tay-Sachs?

      Hepatosplenomegaly reflects visceral storage, which can occur in Sandhoff disease but is not typical of Tay-Sachs.

2. B. Tay-Sachs disease (Why this does not fit)

   Tay-Sachs causes isolated hexosaminidase A deficiency with preserved hexosaminidase B and typically lacks hepatosplenomegaly.

   Reasoning steps for option B

   1. Why does Tay-Sachs disease come to mind first for this infant?

      Regression, hypotonia, and a cherry-red macula at 6 months are the classic acute infantile GM2 picture.

   2. Which two findings separate this infant from classic Tay-Sachs?

      Hexosaminidase B is also undetectable, and the liver and spleen are enlarged, whereas Tay-Sachs preserves HEX B and lacks organomegaly.

3. C. Infantile neurovisceral ASMD (Why this does not fit)

   ASMD can cause organomegaly, neurodegeneration, and a cherry-red macula, but the defining biochemical defect is acid sphingomyelinase deficiency rather than loss of both hexosaminidases.

   Reasoning steps for option C

   1. Why does infantile neurovisceral ASMD fit much of this presentation?

      It combines neurodegeneration, a cherry-red macula, and hepatosplenomegaly, just as this infant shows.

   2. Which laboratory result points away from ASMD?

      ASMD is acid sphingomyelinase deficiency; it does not abolish both hexosaminidase activities.

4. D. Krabbe disease (Why this does not fit)

   Krabbe disease causes GALC-deficient demyelination with neuropathy and globoid cells, not combined hexosaminidase A and B deficiency.

   Reasoning steps for option D

   1. Why might Krabbe disease be considered in an infant with regression and hypotonia?

      Infantile Krabbe disease is another rapidly progressive lysosomal neurodegeneration of the first year.

   2. Which findings are not explained by Krabbe disease?

      Krabbe disease is GALC deficiency and does not cause a cherry-red macula with organomegaly and absent hexosaminidase A and B.

Takeaway: When both hexosaminidase A and B activities are deficient, Sandhoff disease is the GM2 gangliosidosis that fits, especially when visceral storage is present.

Case sources: ^{[1] [2]}

1. A. The improved blood counts prove the GBA1 diagnosis was incorrect (Why this does not fit)

   Visceral response is expected when systemic Gaucher storage improves and does not invalidate a confirmed GBA1 disorder.

   Reasoning steps for option A

   1. Why might someone question the original diagnosis when two organ systems respond differently?

      A discordant response can make clinicians wonder whether the first diagnosis was wrong.

   2. What does the visceral improvement actually show?

      Shrinking spleen and rising blood counts are the expected response of Gaucher macrophage storage to enzyme replacement, so they support the GBA1 diagnosis.

2. B. Neuronopathic symptoms imply a new HEXA deficiency acquired during treatment (Why this does not fit)

   A second inherited lysosomal enzyme deficiency is not suggested; neuronopathic Gaucher disease itself can progress in the central nervous system.

   Reasoning steps for option B

   1. Why could a second lysosomal disease seem to explain new neurologic signs?

      A HEXA disorder causes progressive neurologic decline, which could seem to explain worsening ataxia.

   2. What is the simpler explanation for the gaze and ataxia findings?

      Neuronopathic Gaucher disease itself causes horizontal gaze initiation problems and ataxia, and nothing in the stem suggests a second inherited deficiency.

3. C. Systemic enzyme reaches visceral macrophage compartments better than the central nervous system (Best answer)

   Intravenous Gaucher enzyme replacement can improve systemic manifestations, but it does not adequately correct central nervous system storage across the blood-brain barrier.

   Reasoning steps for option C

   1. Which two responses in this boy do not move together?

      Splenomegaly and cytopenias improve, while horizontal gaze initiation and truncal ataxia worsen.

   2. Why does intravenous enzyme treat the spleen but not the brain?

      Circulating enzyme reaches macrophages in spleen, liver, and marrow but crosses the blood-brain barrier poorly, so central neuronopathic storage continues.

4. D. Splenic shrinkage directly causes cerebellar degeneration by lowering circulating lipids (Why this does not fit)

   There is no mechanism linking therapeutic splenic response to cerebellar injury; the relevant limitation is tissue access of systemic enzyme.

   Reasoning steps for option D

   1. Why might someone link the spleen response to the new neurologic findings?

      The two changes happened during the same treatment period, which invites a causal link.

   2. Is there a mechanism connecting a smaller spleen to cerebellar injury?

      No; the neurologic progression reflects untreated central Gaucher storage behind the blood-brain barrier, not the visceral response.

Takeaway: A systemic response does not prove central nervous system correction; neuronopathic Gaucher disease can progress because intravenous enzyme has limited brain access.

Case sources: ^[3]

1. A. Exclude Fabry disease because a heterozygous female must have low enzyme activity (Why this does not fit)

   α-galactosidase A activity can fall within the normal range in heterozygous females, so enzyme activity alone cannot reliably exclude Fabry disease.

   Reasoning steps for option A

   1. Why is it tempting to exclude Fabry disease when enzyme activity is 72% of the mean?

      In affected males, near-normal α-galactosidase A activity effectively excludes classic Fabry disease.

   2. Why does that rule fail in a heterozygous woman?

      Random X inactivation can leave her leukocyte α-galactosidase A activity in the normal range even when she has Fabry disease.

2. B. Test for the familial pathogenic GLA variant and assess organ involvement (Best answer)

   A symptomatic woman with an affected brother requires molecular evaluation for the known familial GLA variant; if present, cardiac, renal, neurologic, and other organ assessment is appropriate.

   Reasoning steps for option B

   1. Which findings make Fabry disease likely in this woman despite the enzyme result?

      Burning foot pain, cornea verticillata, left ventricular hypertrophy, and a brother with confirmed Fabry disease point to a heterozygous woman with Fabry disease.

   2. Why is testing for the familial GLA variant the decisive next step?

      Genetic testing for the known family variant confirms the diagnosis when enzyme activity cannot, and a positive result leads to organ assessment.

3. C. Measure hexosaminidase B because Fabry disease in women converts to Sandhoff disease (Why this does not fit)

   Fabry and Sandhoff are distinct inherited disorders; sex does not change GLA deficiency into HEXB deficiency.

   Reasoning steps for option C

   1. Why might a hexosaminidase test come up in a lysosomal work-up?

      Hexosaminidase assays are a standard part of lysosomal enzyme panels.

   2. Is there any link between Fabry disease and hexosaminidase B?

      No; Fabry disease is caused by GLA variants on the X chromosome, while hexosaminidase B deficiency reflects HEXB variants in Sandhoff disease.

4. D. Diagnose an asymptomatic carrier state because women cannot develop Fabry organ disease (Why this does not fit)

   Heterozygous females can develop clinically important Fabry manifestations, including cardiac and renal disease.

   Reasoning steps for option D

   1. Why might this woman be labeled an asymptomatic carrier?

      Older teaching treated women with X-linked disease as carriers without organ disease.

   2. Which findings show that she already has organ involvement?

      Cornea verticillata, neuropathic pain, and left ventricular hypertrophy are Fabry manifestations, not a carrier-only state.

Takeaway: Normal or near-normal α-galactosidase A activity does not exclude Fabry disease in a heterozygous female; molecular testing is central when a familial GLA variant is known.

Case sources: ^[4]

1. A. MPS I (Why this does not fit)

   MPS I shares heparan and dermatan sulfate storage but is caused by α-L-iduronidase deficiency and commonly includes corneal clouding in the more severe phenotype.

   Reasoning steps for option A

   1. Why does MPS I compete closely with this boy's presentation?

      MPS I shares coarse features, hearing loss, joint stiffness, organomegaly, and heparan plus dermatan sulfate excess.

   2. Which two findings separate MPS I from the stem?

      The corneas are clear and the measured deficient enzyme is iduronate-2-sulfatase, not α-L-iduronidase.

2. B. MPS III (Why this does not fit)

   MPS III can cause progressive behavioral and neurologic disease, but its enzyme defects are in heparan-sulfate degradation and it does not explain the measured iduronate-2-sulfatase deficiency.

   Reasoning steps for option B

   1. Why could MPS III be considered in a boy with progressive behavioral difficulty?

      Sanfilippo syndrome is dominated by behavioral and neurologic decline.

   2. Which laboratory findings exclude MPS III?

      MPS III stores heparan sulfate alone and is caused by heparan-degrading enzymes, whereas this boy has dermatan sulfate excess and low iduronate-2-sulfatase.

3. C. MPS VII (Why this does not fit)

   MPS VII is β-glucuronidase deficiency and can cause skeletal-visceral disease, but the enzyme assay here directly identifies a different mucopolysaccharidosis.

   Reasoning steps for option C

   1. Why could MPS VII fit a child with coarse features and organomegaly?

      Sly syndrome also combines skeletal disease, coarse facies, and hepatosplenomegaly.

   2. Which enzyme result makes MPS VII unlikely?

      MPS VII is β-glucuronidase deficiency, while this assay directly shows iduronate-2-sulfatase deficiency.

4. D. MPS II (Best answer)

   Low iduronate-2-sulfatase with heparan and dermatan sulfate storage, a male patient, coarse features, organomegaly, and typically clear corneas identifies MPS II.

   Reasoning steps for option D

   1. Which findings narrow this boy's disorder to MPS I or II?

      Coarse features, hearing loss, joint stiffness, hepatosplenomegaly, and heparan plus dermatan sulfate excess fit either MPS I or MPS II.

   2. Which findings then confirm MPS II?

      Clear corneas on slit-lamp examination and iduronate-2-sulfatase at 2% of reference in a boy identify X-linked MPS II.

Takeaway: In a mucopolysaccharidosis with heparan plus dermatan sulfate storage, iduronate-2-sulfatase deficiency and usually clear corneas identify MPS II.

Case sources: ^[10]

1. A. Glycosphingolipid storage with secondary myocardial injury and fibrosis (Best answer)

   Fabry disease causes glycosphingolipid storage in the heart, and chronic cellular injury can lead to myocardial fibrosis that appears as late gadolinium enhancement.

   Reasoning steps for option A

   1. Which background findings establish Fabry cardiomyopathy as the frame for this MRI?

      A pathogenic GLA variant, angiokeratomas, reduced kidney function, and left ventricular hypertrophy despite controlled blood pressure fit Fabry cardiac involvement.

   2. What does inferolateral mid-wall late gadolinium enhancement represent here?

      It marks myocardial fibrosis that develops after chronic glycosphingolipid storage injures cardiomyocytes, a typical Fabry pattern.

2. B. Transthyretin amyloid deposition caused directly by GLA deficiency (Why this does not fit)

   Transthyretin amyloidosis can also cause ventricular thickening, but a confirmed GLA disorder with Fabry manifestations points to Fabry cardiomyopathy rather than a direct conversion to amyloidosis.

   Reasoning steps for option B

   1. Why does transthyretin amyloidosis come up with unexplained ventricular thickening?

      Amyloid cardiomyopathy is a leading cause of hypertrophy with late gadolinium enhancement in adults.

   2. Why is amyloid the wrong mechanism in this man?

      GLA deficiency causes glycosphingolipid storage, not transthyretin deposition, and his confirmed Fabry features explain the hypertrophy.

3. C. Lysosomal glycogen accumulation from acid α-glucosidase deficiency (Why this does not fit)

   That mechanism causes Pompe disease, not Fabry disease, and the patient has a confirmed pathogenic GLA variant.

   Reasoning steps for option C

   1. Why might lysosomal glycogen storage be considered for hypertrophic cardiomyopathy?

      Pompe disease is a lysosomal disorder that can cause marked ventricular hypertrophy.

   2. Which finding makes Pompe the wrong mechanism?

      The confirmed pathogenic GLA variant with angiokeratomas and kidney disease identifies Fabry disease, which stores glycosphingolipid rather than glycogen.

4. D. Myocardial iron overload from defective hepcidin signaling (Why this does not fit)

   Iron overload can cause cardiomyopathy, but it does not explain the GLA variant, angiokeratomas, or Fabry-specific multisystem pattern.

   Reasoning steps for option D

   1. Why could iron overload be considered in a cardiomyopathy with abnormal MRI?

      Myocardial iron deposition is a treatable cause of cardiomyopathy that cardiac MRI is used to detect.

   2. Which findings does iron overload fail to explain?

      Iron overload does not explain the GLA variant, angiokeratomas, or Fabry kidney disease.

Takeaway: Fabry cardiomyopathy reflects glycosphingolipid storage plus downstream myocardial injury; late gadolinium enhancement can indicate fibrosis rather than simple storage alone.

Case sources: ^[4]

1. A. The normalized blood counts prove the bone pain is unrelated to Gaucher disease (Why this does not fit)

   Hematologic response does not establish full skeletal recovery; Gaucher bone disease can remain clinically important even when blood counts and organ volumes improve.

   Reasoning steps for option A

   1. Why might normal blood counts suggest that the hip pain is unrelated to Gaucher disease?

      Blood counts and organ size are the easiest markers of Gaucher response, so their improvement can seem to signal full control.

   2. What do the MRI findings show about the skeleton?

      Femoral-head osteonecrosis and persistent abnormal marrow signal show active or residual Gaucher bone disease despite the hematologic response.

2. B. The findings show conversion from Gaucher disease to Pompe disease (Why this does not fit)

   Osteonecrosis and marrow infiltration are recognized Gaucher skeletal problems and do not indicate acquisition of a separate GAA deficiency.

   Reasoning steps for option B

   1. Why might Pompe disease be considered when a treated lysosomal patient develops new symptoms?

      Pompe disease is another lysosomal disorder, and new symptoms during treatment can prompt a search for a second diagnosis.

   2. Why is a separate GAA deficiency unnecessary to explain this boy's findings?

      Osteonecrosis and marrow infiltration are recognized Gaucher skeletal complications and do not suggest muscle glycogen storage.

3. C. Skeletal injury can persist or declare itself despite a strong visceral and hematologic response (Best answer)

   Bone compartments and established structural injury do not necessarily recover in parallel with blood counts and organ size, so focal pain requires direct skeletal evaluation.

   Reasoning steps for option C

   1. Which responses show good systemic control in this patient?

      Hemoglobin, platelet count, liver size, and spleen size have all substantially improved on enzyme replacement.

   2. Why can osteonecrosis still appear after such a response?

      Bone and marrow injury responds more slowly and established infarction is structural, so focal pain needs direct skeletal evaluation.

4. D. Enzyme replacement should be stopped because any residual marrow signal proves toxicity (Why this does not fit)

   Residual skeletal abnormalities do not by themselves demonstrate treatment toxicity; the new focal symptom requires assessment of Gaucher bone disease and its complications.

   Reasoning steps for option D

   1. Why might a persistent marrow abnormality be blamed on the treatment?

      A new problem during long-term therapy can raise concern for drug toxicity.

   2. What does the residual marrow signal most likely represent?

      It reflects Gaucher marrow infiltration that improves slowly, not toxicity, and stopping therapy would remove systemic control.

Takeaway: Gaucher response is compartment specific: improved cytopenias and organ size do not exclude persistent marrow disease, infarction, or osteonecrosis.

Case sources: ^[3]

1. A. New loss of both hexosaminidase A and B activities (Why this does not fit)

   Hexosaminidase activities define GM2 gangliosidoses and are not expected to disappear when NPC1-dependent lipid trafficking is restored.

   Reasoning steps for option A

   1. Why might hexosaminidase activity change seem possible after correcting a lysosomal defect?

      Many lysosomal enzymes share the same compartment, so changing one lysosomal protein may seem to affect the others.

   2. Why would restoring NPC1 not create a new hexosaminidase loss?

      Hexosaminidase A and B are HEXA and HEXB products, and restoring the NPC1 transport protein does not alter their genes or activity.

2. B. Improved export and intracellular handling of lysosomal cholesterol (Best answer)

   NPC1 is central to intracellular lipid trafficking, so restoring functional NPC1 should improve handling of unesterified cholesterol that had accumulated in the late endosomal-lysosomal compartment.

   Reasoning steps for option B

   1. What is the primary defect in these NPC1 fibroblasts?

      Lack of functional NPC1 traps unesterified cholesterol in late endosomes and lysosomes because cholesterol export is impaired.

   2. What should happen once functional NPC1 is restored?

      Cholesterol export from the late endosomal-lysosomal compartment should improve, reducing the stored unesterified cholesterol.

3. C. Increased sphingomyelin storage from loss of acid sphingomyelinase (Why this does not fit)

   Acid sphingomyelinase deficiency is a separate SMPD1 disorder and is not the primary biochemical consequence of restoring NPC1.

   Reasoning steps for option C

   1. Why could sphingomyelin storage seem linked to NPC1 disease?

      NPC shares the Niemann-Pick name with acid sphingomyelinase deficiency.

   2. Why would correcting NPC1 not create sphingomyelinase deficiency?

      Acid sphingomyelinase is encoded by SMPD1, so restoring NPC1 trafficking cannot cause a new SMPD1 enzyme loss.

4. D. Accumulation of lysosomal glycogen in cardiac muscle (Why this does not fit)

   Lysosomal glycogen accumulation is caused by GAA deficiency in Pompe disease and is unrelated to correction of NPC1 trafficking.

   Reasoning steps for option D

   1. Why might glycogen accumulation be offered as a lysosomal outcome?

      Lysosomal glycogen storage is a well-known result of a missing lysosomal enzyme in Pompe disease.

   2. Why is glycogen storage unrelated to NPC1 correction?

      Lysosomal glycogen storage requires acid α-glucosidase deficiency, and NPC1 correction affects cholesterol trafficking only.

Takeaway: NPC1 disease is fundamentally an intracellular lipid-trafficking disorder, so restoring NPC1 should improve lysosomal cholesterol handling rather than replace an acid hydrolase.

Case sources: ^[6]

1. A. Compression of large myelinated motor fibers by lysosomal glycogen (Why this does not fit)

   Normal standard nerve-conduction studies argue against a dominant large-fiber process, and lysosomal glycogen storage is a Pompe mechanism rather than Fabry disease.

   Reasoning steps for option A

   1. Why could a large-fiber compressive process be suspected in severe limb pain?

      Nerve compression is a common cause of limb pain, and storage disorders can enlarge tissues.

   2. Which test results argue against a large-fiber process here?

      Normal standard nerve-conduction studies spare large myelinated fibers, while reduced intraepidermal nerve-fiber density shows small-fiber loss.

2. B. Accumulation of GM2 in retinal ganglion cells with optic-nerve injury (Why this does not fit)

   GM2 storage explains Tay-Sachs neuroretinal disease, not heat-triggered distal burning pain with reduced small cutaneous nerve fibers.

   Reasoning steps for option B

   1. Why might retinal GM2 storage be considered in a lysosomal disorder with neurologic symptoms?

      GM2 storage is a familiar cause of neuroretinal injury in the Tay-Sachs family of disorders.

   2. Why does GM2 storage not explain this pain pattern?

      Heat- and exercise-triggered burning pain with reduced cutaneous small fibers is a Fabry small-fiber problem, not a retinal ganglion-cell disorder.

3. C. Inflammatory demyelination of central white matter from sulfatide loss (Why this does not fit)

   Metachromatic leukodystrophy causes central and peripheral demyelination, but this patient has a confirmed Fabry phenotype and selective small-fiber loss.

   Reasoning steps for option C

   1. Why might a demyelinating leukodystrophy be considered for neuropathic pain?

      MLD causes both central and peripheral demyelination.

   2. Which findings point away from sulfatide-related demyelination?

      He has confirmed Fabry disease, normal large-fiber conduction, and selective small-fiber loss rather than central white-matter disease.

4. D. Small-fiber neuropathy associated with Fabry glycosphingolipid storage and tissue injury (Best answer)

   Fabry acroparesthesias are linked to small-fiber dysfunction; normal large-fiber conduction with reduced intraepidermal nerve fibers strongly supports this mechanism.

   Reasoning steps for option D

   1. Which features make this a classic Fabry pain pattern?

      Burning hand and foot pain provoked by fever, exercise, and heat in a teenage boy with Fabry disease are typical acroparesthesias.

   2. How do the nerve studies localize the injured fibers?

      Normal conduction studies with reduced intraepidermal nerve-fiber density show small-fiber neuropathy linked to glycosphingolipid storage.

Takeaway: Fabry pain is a small-fiber neuropathic pattern, so standard large-fiber conduction studies may remain normal despite severe burning pain.

Case sources: ^[4]

1. A. Continue organ-specific surveillance after successful transplantation (Best answer)

   Early transplantation can alter severe MPS I outcomes, especially neurologic course, but valvular, skeletal, ocular, airway, and other somatic disease may not fully resolve and still requires surveillance.

   Reasoning steps for option A

   1. What did early transplantation achieve for this girl?

      Transplantation at age 2 stabilized her developmental trajectory, which is its main benefit in severe MPS I.

   2. Why does she still need organ-specific surveillance?

      Donor-derived enzyme reaches valves, joints, and bone poorly, so progressive mitral thickening and joint restriction need ongoing cardiac and orthopedic care.

2. B. Stop cardiac and skeletal follow-up once neurodevelopment has stabilized (Why this does not fit)

   Stabilized neurodevelopment does not establish resolution of valve or skeletal storage; the new findings demonstrate active organ-specific morbidity.

   Reasoning steps for option B

   1. Why might stable development seem to justify ending follow-up?

      Neurodevelopment is often the main reason for early transplantation, so its stabilization can look like complete success.

   2. What do the echocardiogram and joint exam show?

      Progressive mitral-valve thickening and worsening joint restriction show active somatic disease that still needs monitoring.

3. C. Assume the new findings prove the original MPS I diagnosis was incorrect (Why this does not fit)

   Progressive valve and joint disease are compatible with MPS I and can remain important after transplantation.

   Reasoning steps for option C

   1. Why might new valve disease raise doubt about the original diagnosis?

      New organ disease after successful therapy can seem inconsistent with the expected course.

   2. Is valve and joint progression compatible with treated MPS I?

      Yes; valve and skeletal disease can progress after transplantation in MPS I and do not suggest misdiagnosis.

4. D. Reclassify the patient as MPS II because corneal and skeletal disease can persist after transplant (Why this does not fit)

   Transplant response does not change the underlying IDUA disorder into IDS deficiency; MPS II is a different X-linked disease.

   Reasoning steps for option D

   1. Why could MPS II be proposed when somatic disease persists?

      MPS II shares many somatic features with MPS I, including valve and joint disease.

   2. Why does the persisting disease not change the diagnosis?

      Her disorder is α-L-iduronidase deficiency, and a female with MPS I cannot be reclassified as X-linked IDS deficiency because of treatment response.

Takeaway: Hematopoietic stem-cell transplantation can alter severe MPS I disease course but does not eliminate the need for lifelong organ-specific surveillance.

Case sources: ^[9]

1. A. Gaucher disease type 1 (Why this does not fit)

   Type 1 Gaucher disease lacks primary neuronopathic involvement; the ocular motor and cerebellar findings require a neuronopathic classification.

   Reasoning steps for option A

   1. Which findings in this boy suggest Gaucher disease type 1?

      Splenomegaly, thrombocytopenia, bone pain, and low acid β-glucosidase with biallelic GBA1 variants are the systemic Gaucher pattern of type 1.

   2. Which neurologic findings exclude type 1 classification?

      Horizontal saccadic initiation difficulty and progressive ataxia indicate primary neuronopathic disease, which type 1 by definition lacks.

2. B. Infantile neurovisceral ASMD (Why this does not fit)

   ASMD is an SMPD1 acid sphingomyelinase disorder and does not explain the low acid β-glucosidase activity or GBA1 variants.

   Reasoning steps for option B

   1. Why could infantile neurovisceral ASMD seem to fit this boy?

      ASMD also combines splenomegaly with neurologic disease.

   2. Which laboratory results point away from ASMD?

      Low acid β-glucosidase with biallelic GBA1 variants identifies Gaucher disease rather than SMPD1 sphingomyelinase deficiency.

3. C. Gaucher disease type 3 (Best answer)

   Childhood systemic Gaucher findings plus slowly progressive neurologic features such as horizontal saccadic abnormalities and ataxia fit chronic neuronopathic Gaucher disease type 3.

   Reasoning steps for option C

   1. Which findings define chronic neuronopathic Gaucher disease?

      A 9-year-old with systemic Gaucher findings plus slowly progressive horizontal saccadic difficulty and ataxia fits Gaucher disease type 3.

   2. How does the course separate type 3 from type 2?

      Type 2 causes rapid neurodegeneration in infancy, whereas type 3 progresses slowly in childhood, as in this boy.

4. D. Krabbe disease (Why this does not fit)

   Krabbe disease is GALC deficiency with demyelination and neuropathy, not a GBA1 disorder with splenomegaly, cytopenias, and bone disease.

   Reasoning steps for option D

   1. Why might Krabbe disease be proposed for progressive neurologic decline?

      Krabbe disease is another lysosomal neurodegeneration that can present later in childhood.

   2. Which findings exclude Krabbe disease?

      This boy has splenomegaly, thrombocytopenia and bone pain with low acid β-glucosidase, a visceral and marrow pattern that the galactocerebrosidase-deficient leukodystrophy does not produce.

Takeaway: Neurologic findings in a confirmed GBA1 disorder distinguish chronic neuronopathic Gaucher type 3 from non-neuronopathic type 1.

Case sources: ^[3]

1. A. She is only a carrier, so her kidney and heart findings are unrelated to Fabry disease (Why this does not fit)

   Heterozygous females can have clinically important Fabry renal and cardiac disease, so her organ findings cannot be dismissed as unrelated solely because she is female.

   Reasoning steps for option A

   1. Why might this woman be told her findings are carrier-only?

      Older teaching treated heterozygous women with X-linked disease as carriers without significant organ disease.

   2. Which findings show that her Fabry disease is clinically active?

      Albuminuria and left ventricular hypertrophy are Fabry kidney and heart manifestations in a heterozygous woman.

2. B. Plan pregnancy around her organ involvement and X-linked transmission risk (Best answer)

   A heterozygous woman can be clinically affected. Preconception counseling therefore considers maternal cardiac and renal status along with the 50% chance of transmitting her GLA variant in each pregnancy.

   Reasoning steps for option B

   1. Which two issues does her preconception counseling need to cover?

      Her own renal and cardiac Fabry disease, which affects pregnancy planning, and the chance of passing on the variant.

   2. What is the transmission risk for each pregnancy?

      As a heterozygous mother, she has a 50% chance of transmitting the GLA variant to each child, son or daughter.

3. C. All sons will inherit the variant because an affected woman transmits her X chromosome to every son (Why this does not fit)

   A heterozygous mother has two X chromosomes, so each child has a 50% chance of receiving the X chromosome carrying the familial GLA variant.

   Reasoning steps for option C

   1. Why might someone expect every son to inherit her variant?

      Every son does receive an X chromosome from his mother.

   2. Why is transmission to sons not certain?

      She has one variant and one normal X chromosome, so each son has a 50% chance of receiving the variant X.

4. D. All daughters will be unaffected because females cannot express Fabry disease (Why this does not fit)

   Daughters who inherit the variant can have Fabry manifestations, and female sex does not guarantee an asymptomatic course.

   Reasoning steps for option D

   1. Why might daughters be assumed to be protected?

      Daughters also receive a normal X chromosome from an unaffected father, which could seem protective.

   2. What can happen in a daughter who inherits the variant?

      Like her mother, a heterozygous daughter can develop Fabry manifestations because of variable X inactivation.

Takeaway: Fabry disease in heterozygous females can be clinically significant; reproductive counseling should address both maternal organ disease and X-linked transmission.

Case sources: ^[4]

1. A. HEXA, which removes the α subunit used only by hexosaminidase A (Why this does not fit)

   HEXA deficiency primarily abolishes hexosaminidase A while hexosaminidase B remains functional, producing the Tay-Sachs enzyme pattern rather than combined loss.

   Reasoning steps for option A

   1. Why is HEXA the first gene that comes to mind in GM2 disease?

      HEXA deficiency causes Tay-Sachs disease, the best-known GM2 gangliosidosis with regression and a cherry-red macula.

   2. Which enzyme result does a HEXA defect fail to explain?

      HEXA encodes the α subunit found only in hexosaminidase A, so it spares hexosaminidase B, which is 2% here.

2. B. GBA1, which encodes lysosomal acid β-glucosidase (Why this does not fit)

   GBA1 deficiency causes Gaucher disease and does not directly determine hexosaminidase A and B activity.

   Reasoning steps for option B

   1. Why might GBA1 come up in an infant with hepatosplenomegaly and regression?

      Type 2 Gaucher disease causes infantile neurodegeneration with organomegaly.

   2. Does GBA1 affect hexosaminidase activity?

      No; GBA1 encodes acid β-glucosidase, so it cannot explain the paired loss of hexosaminidase A and B.

3. C. SMPD1, which encodes acid sphingomyelinase (Why this does not fit)

   SMPD1 deficiency causes ASMD with sphingomyelin storage but does not explain severe loss of both hexosaminidase activities.

   Reasoning steps for option C

   1. Why does SMPD1 fit several features of this child?

      Infantile neurovisceral ASMD also causes regression, a cherry-red macula, and hepatosplenomegaly.

   2. Which enzyme pattern points away from SMPD1?

      SMPD1 encodes acid sphingomyelinase and does not reduce hexosaminidase A and B.

4. D. HEXB, which encodes the β subunit shared by hexosaminidase A and B (Best answer)

   Hexosaminidase A contains α and β subunits, while hexosaminidase B contains β subunits. HEXB deficiency therefore reduces both measured activities and causes Sandhoff disease.

   Reasoning steps for option D

   1. How are hexosaminidase A and B built from subunits?

      Hexosaminidase A is an α-β heterodimer, and hexosaminidase B is a β-β homodimer.

   2. Why does a HEXB defect lower both measured activities?

      HEXB encodes the β subunit shared by both isoenzymes, so its loss reduces hexosaminidase A and B together and causes Sandhoff disease.

Takeaway: Sandhoff disease is caused by HEXB deficiency because the β subunit contributes to both hexosaminidase A and B, explaining the paired enzyme loss.

Case sources: ^[2]

1. A. MPS VII (Best answer)

   MPS VII is caused by β-glucuronidase deficiency and can produce prenatal hydrops plus later skeletal, visceral, respiratory, ocular, and developmental manifestations.

   Reasoning steps for option A

   1. Which history is unusual among the mucopolysaccharidoses?

      Nonimmune hydrops before birth is a recognized presentation of MPS VII.

   2. Which enzyme result confirms MPS VII?

      β-glucuronidase at 2% of reference with high urinary glycosaminoglycans directly identifies Sly syndrome.

2. B. MPS I (Why this does not fit)

   MPS I is α-L-iduronidase deficiency. Its phenotype can overlap, but the measured β-glucuronidase deficiency directly identifies MPS VII.

   Reasoning steps for option B

   1. Why does MPS I fit much of this boy's phenotype?

      Short stature, coarse features, hepatosplenomegaly, respiratory problems, and skeletal deformity all occur in MPS I.

   2. Which laboratory result points away from MPS I?

      MPS I requires α-L-iduronidase deficiency, whereas this boy has measured β-glucuronidase deficiency.

3. C. MPS II (Why this does not fit)

   MPS II is X-linked iduronate-2-sulfatase deficiency; the enzyme result and history of prenatal hydrops fit MPS VII better.

   Reasoning steps for option C

   1. Why could MPS II fit a boy with coarse features and organomegaly?

      MPS II is X-linked, affects mainly boys, and causes a similar somatic phenotype.

   2. Which findings point away from MPS II?

      The deficient enzyme is β-glucuronidase rather than iduronate-2-sulfatase, and prenatal hydrops is a recognized clue to MPS VII.

4. D. MPS III (Why this does not fit)

   MPS III is dominated by progressive neurologic and behavioral disease from heparan-sulfate degradation defects, not β-glucuronidase deficiency with severe skeletal-visceral disease.

   Reasoning steps for option D

   1. Why might MPS III be considered in a child with elevated urinary GAGs?

      Sanfilippo syndrome is a common mucopolysaccharidosis with high urinary glycosaminoglycans.

   2. Which features point away from MPS III?

      MPS III usually has mild somatic disease and heparan-sulfate enzyme defects, while this boy has severe skeletal-visceral disease and β-glucuronidase deficiency.

Takeaway: MPS VII is β-glucuronidase deficiency; prenatal hydrops can precede the later skeletal and visceral storage phenotype.

Case sources: ^[12]

1. A. Acute infantile Tay-Sachs disease with an unusually delayed diagnosis (Why this does not fit)

   The age and slower neurologic course do not fit the acute infantile phenotype, and measurable residual hexosaminidase A activity supports a later-onset HEXA disorder.

   Reasoning steps for option A

   1. Why might this presentation be read as a delayed diagnosis of classic Tay-Sachs disease?

      The low hexosaminidase A with normal hexosaminidase B is the same enzyme pattern seen in infantile Tay-Sachs.

   2. Which features rule out the acute infantile phenotype?

      Onset at age 10 with slower gait ataxia, weakness, and school decline, plus measurable residual hexosaminidase A, fit a later-onset HEXA disorder.

2. B. Sandhoff disease because any GM2 disorder lowers both enzymes (Why this does not fit)

   Sandhoff disease lowers both hexosaminidase A and B; hexosaminidase B is normal here.

   Reasoning steps for option B

   1. Why might Sandhoff disease be considered for a later-onset GM2 picture?

      Sandhoff disease also has later-onset forms with ataxia and weakness.

   2. Why does the hexosaminidase B result rule out a later-onset Sandhoff phenotype?

      Hexosaminidase B is normal, whereas Sandhoff disease lowers both hexosaminidase A and B at any age of onset.

3. C. A later-onset HEXA disorder reflecting residual enzyme activity (Best answer)

   HEXA disorders form a phenotypic continuum. Later neurologic onset with preserved hexosaminidase B and measurable residual hexosaminidase A activity fits a later-onset Tay-Sachs phenotype.

   Reasoning steps for option C

   1. Which clinical features suggest a later-onset GM2 gangliosidosis?

      Progressive gait ataxia, dysarthria, proximal weakness, and cognitive decline in a 10-year-old without organomegaly fit juvenile GM2 disease.

   2. How does residual activity explain the later onset?

      Hexosaminidase A at about 5% of reference with normal hexosaminidase B means a partially active HEXA product, which delays symptoms compared with infantile disease.

4. D. Metachromatic leukodystrophy because ataxia always indicates arylsulfatase A deficiency (Why this does not fit)

   Ataxia is not disease specific. The enzyme pattern directly implicates HEXA, while MLD requires ARSA deficiency with sulfatide-related disease evidence.

   Reasoning steps for option D

   1. Why might MLD be proposed for childhood ataxia and school decline?

      Juvenile MLD causes gait disturbance and declining school performance.

   2. Which result makes MLD unlikely here?

      The enzyme data directly show hexosaminidase A deficiency, and MLD would require arylsulfatase A deficiency with sulfatide evidence.

Takeaway: HEXA deficiency is a continuum: residual activity can shift presentation from acute infantile disease to later neurologic phenotypes.

Case sources: ^[1]

1. A. MPS II (Why this does not fit)

   MPS II can include behavioral and neurologic disease, but it stores both heparan and dermatan sulfate and is caused by iduronate-2-sulfatase deficiency.

   Reasoning steps for option A

   1. Why is MPS II a plausible competitor in a boy with behavioral decline?

      Neuronopathic MPS II causes hyperactivity, behavioral change, and developmental regression in boys.

   2. Which laboratory findings point away from MPS II?

      Only heparan sulfate is increased, and the deficient enzyme is heparan N-sulfatase rather than iduronate-2-sulfatase.

2. B. MPS IIIA (Best answer)

   CNS-predominant decline with relatively mild somatic disease, high heparan sulfate, and heparan N-sulfatase deficiency identifies Sanfilippo syndrome type A.

   Reasoning steps for option B

   1. Which clinical pattern suggests Sanfilippo syndrome?

      Hyperactivity, sleep disturbance, and loss of language with only mild hepatomegaly and joint stiffness is a CNS-predominant MPS pattern.

   2. Which laboratory results identify MPS IIIA specifically?

      Isolated urinary heparan sulfate excess with heparan N-sulfatase at 2% of reference identifies Sanfilippo syndrome type A.

3. C. MPS I (Why this does not fit)

   MPS I often has more prominent somatic and skeletal disease with α-L-iduronidase deficiency and storage of both heparan and dermatan sulfate.

   Reasoning steps for option C

   1. Why could MPS I be considered for an MPS with neurologic decline?

      Severe MPS I combines developmental decline with somatic storage disease.

   2. Which findings point away from MPS I?

      The skeletal disease is much milder than MPS I, dermatan sulfate is not increased, and the measured deficiency is heparan N-sulfatase.

4. D. MPS VII (Why this does not fit)

   MPS VII is β-glucuronidase deficiency and often has prominent skeletal-visceral disease; it does not explain the specific heparan N-sulfatase result.

   Reasoning steps for option D

   1. Why might MPS VII come up in a child with hepatomegaly and elevated GAGs?

      Sly syndrome also produces glycosaminoglycan excess with visceral involvement.

   2. Which findings point away from MPS VII?

      MPS VII usually has prominent skeletal and visceral disease and β-glucuronidase deficiency, not isolated heparan N-sulfatase loss.

Takeaway: Sanfilippo syndrome is a heparan-sulfate degradation disorder in which progressive CNS and behavioral disease can dominate over somatic findings.

Case sources: ^[11]

1. A. Childhood/adult-onset lysosomal acid lipase deficiency (Best answer)

   Later-onset LAL-D can produce hepatic lipid storage, fibrosis, and an atherogenic lipid profile; the low lysosomal acid lipase activity directly supports the diagnosis.

   Reasoning steps for option A

   1. Which findings suggest later-onset lysosomal acid lipase deficiency?

      Hepatomegaly, steatosis with fibrosis, high LDL and triglycerides, and low HDL in an adult fit childhood/adult-onset LAL-D.

   2. What confirms the diagnosis?

      Lysosomal acid lipase activity at 6% of reference directly identifies LAL-D, and the absence of neurologic signs fits the later-onset form.

2. B. Niemann-Pick disease type C (Why this does not fit)

   NPC can include visceral disease, but the characteristic progressive neurologic pattern and NPC1/NPC2 trafficking defect are absent, while lysosomal acid lipase is directly deficient.

   Reasoning steps for option B

   1. Why does NPC enter a differential for lysosomal lipid storage with liver disease?

      NPC also causes lysosomal cholesterol storage and can present with hepatic involvement.

   2. Which findings point away from NPC?

      He has no ataxia or gaze palsy, and his lysosomal acid lipase is directly deficient, which NPC would not cause.

3. C. Familial hypercholesterolemia from an LDL receptor defect (Why this does not fit)

   Familial hypercholesterolemia can markedly increase LDL, but it does not explain low lysosomal acid lipase activity with hepatic storage disease and hypertriglyceridemia.

   Reasoning steps for option C

   1. Why might familial hypercholesterolemia be suspected with an LDL of 182 mg/dL?

      Raised LDL cholesterol is the hallmark of familial hypercholesterolemia.

   2. Which findings does familial hypercholesterolemia not explain?

      It does not cause hepatic fibrosis with steatosis, low lysosomal acid lipase, or marked hypertriglyceridemia, and he has no tendon xanthomas.

4. D. Gaucher disease type 1 (Why this does not fit)

   Gaucher disease can cause hepatosplenomegaly and bone disease, but it is caused by acid β-glucosidase deficiency rather than low lysosomal acid lipase with this lipid pattern.

   Reasoning steps for option D

   1. Why could Gaucher disease type 1 be considered in an adult with hepatomegaly?

      Type 1 Gaucher disease causes visceral enlargement in adults without neurologic signs.

   2. Which laboratory results point away from Gaucher disease?

      Gaucher disease requires acid β-glucosidase deficiency; this man has low lysosomal acid lipase with an atherogenic lipid profile.

Takeaway: Childhood/adult-onset LAL-D combines lysosomal acid lipase deficiency with progressive liver disease and characteristic dyslipidemia; older CESD terminology refers to this spectrum.

Case sources: ^[14]