A 32-year-old man with a 5-year history of inflammatory bowel disease presents with recurrent perianal fistulas, skin tags, and a stricture of the terminal ileum on colonoscopy. Which histopathological feature would best distinguish his condition from ulcerative colitis?

Explanation

## Distinguishing Crohn Disease from Ulcerative Colitis: Transmural Inflammation and Granulomas ### Key Histopathological Differences **Key Point:** Transmural (full-thickness) inflammation with non-caseating granulomas is the hallmark feature that distinguishes Crohn disease from ulcerative colitis, which is limited to the mucosa and submucosa. ### Comparative Pathology Table | Feature | Crohn Disease | Ulcerative Colitis | | --- | --- | --- | | **Depth of inflammation** | Transmural (all layers) | Mucosal and submucosal only | | **Granulomas** | Non-caseating (30–50% of cases) | Absent | | **Distribution** | Skip lesions (patchy) | Continuous, starts at rectum | | **Fistulas/Strictures** | Common | Rare | | **Crypt distortion** | Variable | Marked, uniform | | **Crypt abscesses** | Present but less prominent | Prominent feature | ### Clinical Correlations **Clinical Pearl:** The transmural nature of Crohn disease explains the patient's perianal fistulas and stricture formation—these complications arise from deep inflammation penetrating through all bowel layers, a feature impossible in ulcerative colitis where inflammation is superficial. **High-Yield:** Non-caseating granulomas are found in 30–50% of Crohn disease specimens and are virtually diagnostic when present; their absence does not exclude Crohn disease. Granulomas are NEVER seen in ulcerative colitis. ### Why Transmural Inflammation Matters 1. **Fistula formation** — transmural penetration allows communication between bowel loops and skin 2. **Strictures** — fibrosis from deep inflammation causes luminal narrowing 3. **Skip lesions** — patchy transmural involvement creates discontinuous disease **Mnemonic: TRANSMURAL** — **T**ransmural, **R**ecurrent fistulas, **A**ll layers involved, **N**on-caseating granulomas, **S**trictures, **M**ultiple skip lesions, **U**niform crypt distortion absent, **R**ectum not always first, **A**bscesses less prominent, **L**imited to mucosa (NO—this is UC). [cite:Robbins 10e Ch 17]