A 32-year-old man with a 6-year history of inflammatory bowel disease presents with chronic diarrhea and abdominal pain. Colonoscopy reveals segmental involvement of the colon with areas of normal mucosa between diseased segments. Histology shows transmural inflammation with non-caseating granulomas. What is the most common site of involvement in this patient's condition?

Explanation

## Most Common Site of Crohn Disease **Key Point:** The terminal ileum and right colon (ileocolic region) represent the most frequent site of involvement in Crohn disease, occurring in approximately 40–50% of cases. ### Anatomical Distribution in Crohn Disease | Site | Frequency | Pattern | | --- | --- | --- | | **Terminal ileum + right colon** | 40–50% | Most common | | **Colon alone** | 20–25% | Second most common | | **Small bowel alone** | 15–20% | Jejunum/ileum | | **Rectum + sigmoid** | 5–10% | Least common | | **Anus/perianal** | Variable | Associated complications | ### Why Terminal Ileum? **High-Yield:** The terminal ileum is the most common site because: 1. It is the narrowest part of the small bowel — predisposing to stasis and bacterial overgrowth 2. It has the highest concentration of Peyer's patches and lymphoid tissue 3. The ileocecal valve creates a natural transition zone with altered mucosal immunity ### Distinguishing Feature: Skip Lesions **Clinical Pearl:** Crohn disease characteristically shows **skip lesions** — discontinuous areas of inflammation separated by normal mucosa (as described in the vignette). This segmental pattern is pathognomonic and reflects the transmural nature of the disease. ### Contrast with Ulcerative Colitis **Warning:** Do not confuse with ulcerative colitis, which: - Always begins at the rectum and extends proximally in a **continuous** fashion - Never involves the small bowel - Never shows skip lesions - Is limited to the mucosa and submucosa (not transmural)