A 28-year-old woman with Crohn's disease undergoes colonoscopy for assessment of disease activity. Histopathology shows transmural inflammation with non-caseating granulomas. Which is the most common site of involvement in Crohn's disease?

Explanation

## Site of Involvement in Crohn's Disease **Key Point:** Crohn's disease most commonly affects the terminal ileum and ileocecal region, with characteristic skip lesions and transmural inflammation. ### Distribution Pattern in Crohn's Disease | Pattern | Frequency | Characteristics | |---------|-----------|------------------| | **Ileocolitis** (terminal ileum + colon) | 45–50% | Most common; ileocecal involvement typical | | **Ileitis alone** (small bowel) | 25–30% | Terminal ileum predominantly affected | | **Colitis alone** | 15–20% | Colon only, sparing terminal ileum | | **Gastroduodenal** | 5–10% | Upper GI involvement; less common | ### Key Distinguishing Features **High-Yield:** The **terminal ileum and ileocecal junction** is the single most common site, involved in approximately 70% of all Crohn's disease cases. **Clinical Pearl:** Crohn's disease can affect ANY part of the GI tract from mouth to anus, but the terminal ileum is the "signature" site. The presence of: - Non-caseating granulomas (60% of cases) - Transmural inflammation (full thickness) - Skip lesions (discontinuous patches) - Cobblestone mucosa all point to Crohn's disease with ileocecal predominance. **Mnemonic:** **ILEOCECAL** — the most common site in Crohn's disease (remember: **I**leum is the **I**nitial and **I**ntense site). [cite:Robbins 10e Ch 17]