A 28-year-old woman with Crohn's disease undergoes colonoscopy for evaluation of persistent abdominal pain and diarrhea. The endoscopist notes patchy areas of inflammation with intervening normal mucosa. Which of the following is the most common site of involvement in Crohn's disease?

Explanation

## Sites of Involvement in Crohn's Disease **Key Point:** The terminal ileum and proximal colon (ileocolonic region) is the most common site of involvement in Crohn's disease, accounting for approximately 40–50% of cases. ### Distribution of Crohn's Disease by Site | Site | Frequency | Pattern | |---|---|---| | Terminal ileum + proximal colon (ileocolonic) | 40–50% (most common) | Skip lesions, cobblestone mucosa | | Small bowel alone (jejunum/ileum) | 20–30% | Patchy involvement | | Colon alone | 15–20% | Can mimic UC but with skip lesions | | Rectosigmoid alone | 5–10% | Rare | | Stomach/duodenum (gastroduodenal CD) | 5% | Rare, associated with fistulizing disease | | Pan-enteric (mouth to anus) | <5% | Rare | **High-Yield:** The ileocolonic junction is a "watershed" area with reduced blood supply and is the most vulnerable to transmural inflammation in Crohn's disease. **Clinical Pearl:** The presence of skip lesions (patchy inflammation with normal intervening mucosa) is a hallmark of Crohn's disease and distinguishes it from ulcerative colitis, which shows continuous inflammation. **Mnemonic:** **CRISP** — Common sites in Crohn's - **C**olon (with terminal ileum) - **R**ectosigmoid (rare alone) - **I**leocolonic junction (most common) - **S**mall bowel (jejunum/ileum) - **P**anenteric (rare) **Warning:** Do not confuse "ileocolonic" with "entire colon." Ileocolonic CD involves the terminal ileum AND proximal colon, not the entire colon. Pancolitis (entire colon) is less common and occurs in only 15–20% of CD cases.