In Crohn disease, the pattern of intestinal involvement is best described as which of the following?

## Pattern of Bowel Involvement in Crohn Disease **Key Point:** Crohn disease characteristically shows **skip lesions** — discontinuous segments of inflamed bowel separated by normal-appearing mucosa. This segmental, non-contiguous pattern is a hallmark distinguishing feature. **High-Yield:** Skip lesions are pathognomonic for Crohn disease. The inflammation is also **transmural** (full-thickness), affecting all layers of the bowel wall, which explains complications like fistulas and strictures. ### Anatomical Distribution in Crohn Disease - **Terminal ileum** — most common site (40% of cases) - **Colon** — 20% of cases - **Small bowel** — 30% of cases - **Ileocolonic** — 50% of cases (combined small and large bowel) - **Any part of GI tract** — from mouth to anus (rare: gastric, duodenal, esophageal involvement) **Clinical Pearl:** The presence of skip lesions on colonoscopy or imaging (CT/MR enterography) strongly suggests Crohn disease. Normal mucosa between inflamed segments is a red flag for Crohn, not ulcerative colitis. **Mnemonic:** **SKIP** — **S**egmental, **K**eep transmural, **I**nflammation patchy, **P**attern discontinuous — the hallmark of Crohn disease. ### Contrast with Ulcerative Colitis | Aspect | Crohn Disease | Ulcerative Colitis | | --- | --- | --- | | **Pattern** | Segmental (skip lesions) | Continuous | | **Depth** | Transmural | Mucosa/submucosa only | | **Rectum involved** | Not always | Always | | **Proximal extent** | Can reach mouth/anus | Limited to colon | | **Fistulas/strictures** | Common | Rare |