Biochem · Metabolism

Lysosomal Storage Diseases

One missing enzyme. Substrate piles up. Cells fail. Every LSD follows the same broken recycling chain.

Opening Challenge

An 8-month-old Ashkenazi Jewish boy is brought in because his parents say he stopped rolling over. He has an exaggerated startle response to sound. Fundoscopic exam shows a cherry-red spot at the macula. Spleen and liver are normal size.

Gaucher disease
Tay-Sachs disease
Niemann-Pick disease type A
Krabbe disease
Tay-Sachs disease. The triad is the board anchor: Ashkenazi infant + cherry-red spot + NO hepatosplenomegaly. Hexosaminidase A is missing, so GM2 ganglioside accumulates in neurons. Gaucher has massive HSM but no cherry-red spot. Niemann-Pick A has cherry-red spot AND HSM. Krabbe causes neurodegeneration through globoid cells, not ganglioside buildup.
01 · One Broken Step

How LSDs Work

Every lysosomal storage disease follows the same chain. Learn this once, apply it everywhere.

Mechanism · Animated

The Lysosome Recycling Failure

Lysosomes are the cell's recycling centers. They use acid hydrolase enzymes to break down worn-out macromolecules. When one enzyme is missing, its substrate piles up inside the lysosome. The cell swells. Organ function fails. Death follows a predictable timeline.

Lysosome Recycling · Click to Play

ACT 1 · NORMAL RECYCLING VESICLE (cargo) LYSOSOME ✂ enzyme active RECYCLED products out Substrate broken down · cell healthy Tap to see what happens when the enzyme is missing ACT 2 · ENZYME MISSING VESICLE (cargo) LYSOSOME ✗ enzyme ABSENT 🚫 Substrate ACCUMULATES · cell swells · organ fails Tap again to reset

Tap the diagram or buttons to switch acts

The Pattern: Lysosomes are cellular recycling centers. They use acid hydrolase enzymes to break down worn-out macromolecules. If one enzyme is missing or dysfunctional, its substrate accumulates inside the lysosome. The cell swells. Organ function fails. Death follows a predictable timeline.

Inheritance pattern

Most LSDs

Autosomal Recessive

Two bad copies of the enzyme gene needed. Both parents are silent carriers. Includes Tay-Sachs, Gaucher, Niemann-Pick, Hurler, Krabbe, Pompe.

X-Linked Exceptions

Fabry and Hunter

Two LSDs break the AR rule: Fabry (alpha-galactosidase A) and Hunter MPS II (iduronate sulfatase). Males affected. Females are carriers who may show mild features.

Shared Mechanism

High-Yield Demographics

Ashkenazi Jewish: Tay-Sachs, Gaucher (type 1), Niemann-Pick (type A). The board loves these. Any Ashkenazi vignette with a sick infant = think LSD first.

Board shortcut: LSD substrate = what the enzyme was supposed to eat. Missing glucocerebrosidase = glucocerebroside piles up (Gaucher). Missing hexosaminidase A = GM2 ganglioside piles up (Tay-Sachs). The name of the disease often hints at the substrate: sphingomyelinase = sphingomyelin (Niemann-Pick).
02 · The Big Five

Sphingolipidoses

Tap each card to reveal enzyme, substrate, and the board-exam clinical triad.

Cherry-red spot in Tay-Sachs
📷 Cherry-Red Spot · tap
Gaucher cells crinkled tissue paper
📷 Gaucher Cells · tap
Angiokeratoma in Fabry disease
📷 Angiokeratoma · tap
Sphingolipidosis Tay-Sachs GM2 ganglioside trap Tap to reveal
Missing enzyme: Hexosaminidase A (Hex A)
Substrate: GM2 ganglioside
Location: Neurons only (NO hepatosplenomegaly)

Cherry-red spot + exaggerated startle + progressive neurodegeneration. Ashkenazi Jewish infants. Death by age 2-4. NO HSM distinguishes it from Niemann-Pick A.
No HSM Cherry-red spot Ashkenazi Hex A deficient
Sphingolipidosis Gaucher Bone-wrecking macrophages Tap to reveal
Missing enzyme: beta-Glucocerebrosidase
Substrate: Glucocerebroside
Location: Macrophages (liver, spleen, bone marrow)

Most common LSD. Gaucher cells = macrophages with "crinkled tissue paper" cytoplasm. Massive HSM, bone crises, avascular necrosis. Erlenmeyer flask femur on X-ray. Type 1 = non-neuronopathic (Ashkenazi). Enzyme replacement (imiglucerase) is first-line.
Massive HSM Erlenmeyer flask Ashkenazi ERT available
Sphingolipidosis Niemann-Pick Foam cells in the organs Tap to reveal
Type A: Sphingomyelinase deficient. Sphingomyelin accumulates. Foam cells in liver/spleen. Cherry-red spot + HSM + neurodegeneration. Ashkenazi. Death by age 3.

Type B: Same enzyme, but spared CNS. Organ-limited.

Type C: NPC1/NPC2 cholesterol trafficking defect (NOT sphingomyelinase). Vertical supranuclear gaze palsy is pathognomonic. Progressive neurodegeneration.
Cherry-red spot (A) HSM (A+B) Vertical gaze palsy (C) Ashkenazi (A)
X-Linked Fabry Burning hands, failing kidneys Tap to reveal
Missing enzyme: alpha-Galactosidase A
Substrate: Globotriaosylceramide (Gb3)
Inheritance: X-linked recessive

Young males. Episodes of severe burning pain in extremities (angiocrises). Angiokeratomas in bathing-suit distribution. Hypohidrosis. Corneal whorling (whorl keratopathy) without vision loss. Progressive renal failure and cardiomyopathy in adulthood.
X-linked Burning pain Angiokeratomas Corneal whorling
Sphingolipidosis Krabbe Globoid cells, white matter Tap to reveal
Missing enzyme: Galactocerebrosidase (GALC)
Substrate: Galactocerebroside
Pathology: Globoid cells (multinucleated macrophages) in white matter

Severe early-infantile neurodegeneration. Irritability and hypertonicity progress to opisthotonos. Peripheral neuropathy. Rapid death in the infantile form. NO HSM. Rare outside of Scandinavian population.
Rapid death Globoid cells Peripheral neuropathy
Cherry-red spot logic: The fovea has a single cell layer and stays red-orange while surrounding retinal ganglion cells swell with substrate and turn white/gray. Seen in: Tay-Sachs (no HSM), Niemann-Pick A (with HSM), and Sandhoff disease. NOT seen in Gaucher, Fabry, or Krabbe.
03 · The Other Vault

Mucopolysaccharidoses + Pompe

Hurler vs. Hunter is a classic board comparison. One detail separates them.

MPS I · Autosomal Recessive

Hurler Syndrome

Missing enzyme: alpha-L-iduronidase
Substrate: Heparan + dermatan sulfate

Gargoyle facies, short stature, joint stiffness. Corneal CLOUDING (hallmark). Intellectual disability, progressive. HSM. Death in first decade.

AR Corneal clouding HSM Intellectual disability

MPS II · X-Linked

Hunter Syndrome

Missing enzyme: Iduronate sulfatase
Substrate: Heparan + dermatan sulfate (same as Hurler)

Similar: coarse facies, HSM, joint stiffness. BUT NO corneal clouding (key board differentiator). Milder CNS involvement. X-linked = males only.

X-linked NO corneal clouding HSM Milder than Hurler

Glycogen in Lysosomes

Pompe Disease

Missing enzyme: Acid maltase (alpha-1,4-glucosidase)
Substrate: Glycogen (trapped in lysosomes, not cytoplasm)

Classic infantile: massive cardiomegaly, hypotonia, "floppy infant," early death. Adult form: proximal myopathy. ERT (alglucosidase) available.

Cardiomegaly Floppy infant ERT available Glycogen in lysosomes
Hurler vs Hunter memory hook: HuRLER has coRneaL cloudIng (both have L). HuNTer has No corneal = No N in clouding. Hunter is also X-liNked = N theme.
Same substrate, different enzyme: Hurler and Hunter BOTH accumulate heparan sulfate + dermatan sulfate. The difference is which enzyme is missing upstream. Corneal clouding and inheritance pattern are the discriminators.
04 · The Board Trap

Eliminate the Imposters

Four diseases. One answer. Reveal clues and eliminate wrong diagnoses until only one remains.

A 9-month-old Ashkenazi Jewish boy presents with progressive neurological decline. He had normal development until 5 months. Parents report he stopped making eye contact and is now mostly limp. He has an exaggerated startle response to sound. Cherry-red spot is seen on fundoscopy.
Tay-Sachs Hex A deficient
Sandhoff Hex A + B deficient
Niemann-Pick A Sphingomyelinase
Niemann-Pick C Cholesterol trafficking
Clue 1: Physical exam shows no hepatomegaly, no splenomegaly. Abdomen is completely soft and normal. Eliminates any disease that requires organomegaly.
Clue 2: No vertical supranuclear gaze palsy. Ocular movements are intact except for the fundoscopic finding. Vertical gaze palsy is pathognomonic for Niemann-Pick C. Absence eliminates it.
Clue 3: Enzyme assay: Hexosaminidase A activity = 0%. Hexosaminidase B activity = 100% (normal). Sandhoff has BOTH Hex A and Hex B deficient. Normal Hex B rules it out.
Diagnosis confirmed: Tay-Sachs disease. No HSM eliminates Niemann-Pick A. No vertical gaze palsy eliminates NP-C. Normal Hex B eliminates Sandhoff. Only Tay-Sachs has isolated Hex A deficiency with cherry-red spot and no organomegaly.
Why Sandhoff matters: Sandhoff disease presents almost identically to Tay-Sachs (cherry-red spot, neurodegeneration, Ashkenazi). The only lab distinction is that Sandhoff lacks BOTH Hex A AND Hex B. Clinically you cannot tell them apart at the bedside.
05 · Fast Reference

Master Comparison Table

Every high-yield LSD in one place. Know the distinguishing column.

Disease Missing Enzyme Substrate Distinguishing Feature Inheritance
Tay-Sachs Hex A GM2 ganglioside Cherry-red spot, NO HSM AR, Ashkenazi
Sandhoff Hex A + B GM2 + globoside Tay-Sachs twin, both Hex A+B gone AR
Gaucher beta-Glucocerebrosidase Glucocerebroside Gaucher cells, Erlenmeyer flask, ERT AR, Ashkenazi
Niemann-Pick A Sphingomyelinase Sphingomyelin Cherry-red spot + HSM AR, Ashkenazi
Niemann-Pick C NPC1/NPC2 protein Cholesterol Vertical gaze palsy (pathognomonic) AR
Fabry alpha-Galactosidase A Ceramide trihexoside (Gb3) Burning pain, angiokeratomas, corneal whorling X-linked
Krabbe Galactocerebrosidase Galactocerebroside Globoid cells, peripheral neuropathy AR
Hurler (MPS I) alpha-L-Iduronidase Heparan + dermatan sulfate Corneal CLOUDING, gargoyle facies AR
Hunter (MPS II) Iduronate sulfatase Heparan + dermatan sulfate NO corneal clouding, X-linked X-linked
Pompe Acid maltase (alpha-glucosidase) Glycogen Cardiomegaly, floppy infant, ERT AR
X-linked list (memorize these two): Fabry and Hunter. Everything else in this table is AR. If a question says "X-linked LSD" the answer is one of these two. If it says "only X-linked MPS" the answer is Hunter alone.
ERT available: Gaucher (imiglucerase), Pompe (alglucosidase alfa), Fabry (agalsidase alfa/beta), Hunter (idursulfase), Hurler (laronidase). Tay-Sachs and Krabbe have NO ERT because CNS involvement makes systemic enzyme replacement ineffective across the blood-brain barrier.
06 · Board Simulation

Apply Everything

Eight clinical questions. No peeking at the table.

Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last reviewed June 2026
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