Step 1: what comes next?
Inflammation screen
CBC, metabolic panel, CRP, and fecal calprotectin support inflammation but do not name its cause.
GI
Crohn's Disease vs Ulcerative Colitis
Map the bowel first, then let depth and complications settle the diagnosis.
- Distinguish Crohn's disease from ulcerative colitis by distribution, depth, histology, imaging, and complications.
- Use fecal calprotectin, endoscopy with biopsy, and cross-sectional imaging without treating any single result as diagnostic.
- Separate induction from maintenance therapy and identify when surgery is curative for colitis or noncurative for Crohn's disease.
Key distinctions
Separate the closest diagnoses
The figure compares the nearest alternatives and highlights the finding that separates them.
Quick check
Contrast the bowel map and wall depth before considering any drug name.
Which pairing is most accurate?
Reason it through
- Where does UC usually begin?At the rectum, extending proximally in a continuous pattern.
- What does transmural inflammation permit?Deep ulcers, strictures, abscesses, and fistulas.
- Does a missing granuloma exclude Crohn's disease?No. Noncaseating granulomas are specific when present but insensitive.
Match complications to wall depth
Transmural injury produces mechanical and penetrating complications; severe colonic inflammation produces systemic toxicity.
Crohn's strictures may contain both active inflammatory edema and fixed fibrosis, so obstruction does not automatically respond to corticosteroids.
Toxic megacolon is classically associated with severe UC but may complicate any severe colitis; systemic toxicity plus colonic dilation demands urgent multidisciplinary care.
Choose the strongest disease association.
Build the two bowel maps
Distribution and depth explain most exam findings before histology is opened.
Crohn's disease can involve any segment from mouth to anus, commonly terminal ileum and colon, with skip areas and rectal sparing.
Ulcerative colitis begins in the rectum and extends continuously through a variable length of colon; inflammation is usually limited to mucosa and superficial submucosa.
Compare the discriminating features.
Crohn's has skip lesions and may involve small bowel; UC is continuous, starts at the rectum, and remains colonic.
Crohn's is transmural; UC is predominantly mucosal, although fulminant colitis can extend deeper.
Noncaseating granulomas away from ruptured crypts favor Crohn's; crypt distortion and crypt abscesses support active chronic colitis and are classic in UC.
Crohn's has deep linear ulcers and cobblestoning; UC has diffuse friability, superficial ulceration, and pseudopolyps.
Granulomas rule in Crohn's more effectively than their absence rules it out.
Calibrate risk instead of memorizing absolutes
Risk rises along gradients of inflammatory burden, colonic extent, duration, family history, and PSC.
Fecal calprotectin is useful for separating inflammatory from functional symptoms and for monitoring, but infection, NSAID exposure, and other intestinal inflammation can elevate it.
Colorectal neoplasia risk is most relevant when Crohn's involves a substantial portion of colon or UC is extensive and longstanding; surveillance timing is individualized by risk.
Some extraintestinal manifestations track bowel activity, such as peripheral arthritis, erythema nodosum, and episcleritis; PSC and axial spondyloarthritis may follow an independent course.
Place each finding on the urgency and risk gradient.
Read symptoms from anatomy
Location and depth predict the clinical pattern better than the label alone.
Terminal ileal Crohn's can cause right lower quadrant pain, bile salt diarrhea, B12 deficiency after extensive disease or resection, and obstructive symptoms from narrowing.
Rectal UC produces urgency, tenesmus, and bloody diarrhea; proximal extension increases inflammatory burden and colorectal cancer risk.
Perianal pain or drainage should trigger an exam for abscess, fissure, skin tags, and fistula, with pelvic imaging when a deep tract is suspected.
Place each finding on the bowel map.
Crohn's: thickened enhancing wall, upstream dilation, creeping fat, or enteroenteric fistula on enterography.
UC: continuous circumferential erythema, loss of vascular pattern, friability, and ulceration.
Crohn's: abscesses and branching fistula tracts that require surgical drainage before immunosuppression escalation.
Severe colitis: dilation with fever, tachycardia, anemia, or metabolic disturbance raises toxic megacolon concern.
Diagnose in layers
No single symptom, biomarker, or scan establishes inflammatory bowel disease.
First confirm an inflammatory syndrome and exclude infection, including stool pathogen testing when clinically indicated.
Then define distribution and severity with ileocolonoscopy and segmental biopsies; add upper endoscopy when upper GI Crohn's is suspected.
Finally map transmural small-bowel and penetrating disease with MR enterography or CT enterography; MRI pelvis is preferred for perianal fistula anatomy.
Order the diagnostic workup.
Step 2: what comes next?
Endoscopic map
Inspect terminal ileum and colon, document continuous versus skip disease, and biopsy involved and uninvolved segments.
Step 3: what comes next?
Cross-sectional map
Use enterography for small-bowel extent, mural thickening, strictures, abscesses, and fistulas.
Step 4: what comes next?
Phenotype and surveillance plan
Record location, behavior, perianal disease, severity, colonic duration, PSC, and other cancer-risk modifiers.
Separate induction, maintenance, and surgery
A drug that controls an acute flare may be unsafe or ineffective as long-term disease control.
For mild-to-moderate ileocecal Crohn's, controlled-release budesonide can induce remission; systemic corticosteroids are reserved for more active disease and are not maintenance therapy.
Moderate-to-severe Crohn's and UC increasingly use effective advanced therapy early, selected by phenotype, prior exposure, comorbidity, safety, and patient preference.
Total proctocolectomy removes UC colitis and its colorectal cancer risk, while Crohn's commonly recurs after resection and surgery treats complications rather than curing the disease.
Reveal the management consequence.
A successful induction is a bridge to a steroid-sparing maintenance strategy, not permission for chronic prednisone.
ACG-supported advanced options include anti-TNF therapy, vedolizumab, ustekinumab, IL-23 therapy, and upadacitinib, chosen for the individual phenotype.
Use an effective biologic or small molecule for induction and continue an effective agent for maintenance; do not use corticosteroids for maintenance.
Drain abscesses, combine perianal fistula care with colorectal surgery, and use resection or strictureplasty for selected fixed complications.
Hospitalize, exclude infection, give IV corticosteroids, assess early for rescue therapy, and involve colorectal surgery before deterioration.
Induction stops the fire; maintenance prevents the next one.
Decisive finding
Pick the discriminator
Choose the finding that separates the closest competing diagnoses.
Which pairing is most accurate?
- Key finding. Contrast the bowel map and wall depth before considering any drug name.
- Answer. Crohn's: skip lesions and transmural inflammation; UC: continuous distal-to-proximal mucosal colitis
- Why. This pairing captures the defining distribution and depth distinction.
- Board rule. Distribution tells you where; depth tells you what complications are possible.
Stage 1 of 3: Overview
Overview
Crohn's Disease vs Ulcerative Colitis
No single symptom, biomarker, or scan establishes inflammatory bowel disease.
Separate the competing diagnoses
Five patients force you to use distribution, depth, complications, biomarkers, and treatment phase rather than buzzwords.
Cross out mimics and highlight the finding that separates the diagnoses. Shuffle to compare a new case order.
A 24-year-old has weight loss, crampy right lower quadrant pain, intermittent obstruction, and a draining perianal opening. MR enterography shows terminal ileal thickening with prestenotic dilation.
Which diagnosis and complication pairing best fits?
Reason it through
- Which finding demonstrates disease extending beyond the mucosa?A fistula is a penetrating transmural complication.
- What does prestenotic dilation imply?A mechanically important stricture, which may be inflammatory, fibrotic, or mixed.
- What must be excluded before escalating immunosuppression for perianal disease?An undrained abscess.
Which finding demonstrates disease extending beyond the mucosa?What does prestenotic dilation imply?
- Which finding demonstrates disease extending beyond the mucosa?A fistula is a penetrating transmural complication.
- What does prestenotic dilation imply?A mechanically important stricture, which may be inflammatory, fibrotic, or mixed.
- What must be excluded before escalating immunosuppression for perianal disease?An undrained abscess.
A 36-year-old with known pancolitis develops fever, heart rate 124/min, abdominal distention, bloody diarrhea, and a transverse colon diameter of 6.5 cm.
What is the best next management frame?
Reason it through
- What two domains define toxic megacolon?Nonobstructive colonic dilation plus systemic toxicity in severe colitis.
- Which common tests or drugs become dangerous?Full colonoscopy, bowel preparation, opioids, and antimotility drugs.
- Why call surgery before medical rescue fails?Delay after deterioration increases perforation, sepsis, and operative risk.
What two domains define toxic megacolon?Which common tests or drugs become dangerous?
- What two domains define toxic megacolon?Nonobstructive colonic dilation plus systemic toxicity in severe colitis.
- Which common tests or drugs become dangerous?Full colonoscopy, bowel preparation, opioids, and antimotility drugs.
- Why call surgery before medical rescue fails?Delay after deterioration increases perforation, sepsis, and operative risk.
A 29-year-old has six weeks of diarrhea and abdominal pain but no bleeding, weight loss, anemia, fever, or nocturnal symptoms. CRP and fecal calprotectin are normal.
Which interpretation is most defensible?
Reason it through
- What is calprotectin actually measuring?Neutrophil-associated intestinal inflammation, not a disease label.
- Which features would change the threshold for endoscopy?Bleeding, anemia, weight loss, fever, nocturnal symptoms, family history, or persistent objective abnormalities.
- What disease category rises when inflammation is absent?A disorder of gut-brain interaction such as IBS, after appropriate limited evaluation.
What is calprotectin actually measuring?Which features would change the threshold for endoscopy?
- What is calprotectin actually measuring?Neutrophil-associated intestinal inflammation, not a disease label.
- Which features would change the threshold for endoscopy?Bleeding, anemia, weight loss, fever, nocturnal symptoms, family history, or persistent objective abnormalities.
- What disease category rises when inflammation is absent?A disorder of gut-brain interaction such as IBS, after appropriate limited evaluation.
A 31-year-old with ileocecal Crohn's enters clinical remission after an eight-week budesonide course. Symptoms recur each time budesonide is tapered.
Which plan best follows induction-versus-maintenance principles?
Reason it through
- Did the induction work?Yes, but remission is not durable off the induction agent.
- What is the hidden diagnosis in repeated tapers?Steroid-dependent disease.
- What should guide the maintenance choice?Objective activity, disease location and behavior, prior therapy, comorbidity, safety, and patient preference.
Did the induction work?What is the hidden diagnosis in repeated tapers?
- Did the induction work?Yes, but remission is not durable off the induction agent.
- What is the hidden diagnosis in repeated tapers?Steroid-dependent disease.
- What should guide the maintenance choice?Objective activity, disease location and behavior, prior therapy, comorbidity, safety, and patient preference.
A 48-year-old with longstanding extensive UC and primary sclerosing cholangitis has minimal bowel symptoms and asks to defer colonoscopy because the disease feels quiet.
Which response is most accurate?
Reason it through
- Which risk factor is especially important here?Primary sclerosing cholangitis.
- Does symptom control equal histologic or neoplastic safety?No. Symptoms correlate imperfectly with inflammation and not with dysplasia.
- What does colectomy change?Total proctocolectomy removes the at-risk colonic mucosa but introduces surgical and pouch considerations.
Which risk factor is especially important here?Does symptom control equal histologic or neoplastic safety?
- Which risk factor is especially important here?Primary sclerosing cholangitis.
- Does symptom control equal histologic or neoplastic safety?No. Symptoms correlate imperfectly with inflammation and not with dysplasia.
- What does colectomy change?Total proctocolectomy removes the at-risk colonic mucosa but introduces surgical and pouch considerations.
Rapid review
Three questions to check
Which pairing is most accurate?
Crohn's: skip lesions and transmural inflammation; UC: continuous distal-to-proximal mucosal colitis. This pairing captures the defining distribution and depth distinction.
Which finding demonstrates disease extending beyond the mucosa?
A fistula is a penetrating transmural complication.
What does prestenotic dilation imply?
A mechanically important stricture, which may be inflammatory, fibrotic, or mixed.
Medically reviewed
Fatima Ali, DO
PGY-1 Resident Physician in Psychiatry
University Hospitals, Columbia
DO from Kansas City University
Resident physician and founding medical reviewer at Bone Wizardry, focused on clinical accuracy, clear diagnostic reasoning, and practical board-oriented teaching across the curriculum.
Languages: English, Urdu
Primary reviewerFull physician profile
Medically reviewed
Sources
- Updated 2025 ACG Clinical Guideline for the Management of Crohn's Disease2025
- The Updated 2025 ACG Guidelines to Manage Adult Ulcerative Colitis Patients2025
Bone Wizardry is a study resource for medical students. It is not medical advice.