## Most Common Site of Crohn's Disease **Key Point:** The terminal ileum and cecum (ileocecal region) are the most frequently involved sites in Crohn's disease, occurring in approximately 40–50% of all cases. ### Pattern of Involvement in Crohn's Disease **High-Yield:** Crohn's disease can involve **any part of the GI tract from mouth to anus**, with a predilection for the ileocecal region. ### Frequency of Site Involvement | Site | Frequency | |------|----------| | **Terminal ileum ± colon (ileocolic)** | 40–50% (most common) | | **Small bowel alone** | 30% | | **Colon alone** | 20% | | **Stomach/duodenum** | 5% | | **Rectum alone** | Rare | ### Why Terminal Ileum is Most Common 1. **Rich lymphoid tissue** — Peyer's patches are abundant in the terminal ileum 2. **Narrow lumen** — Creates a natural site of stasis and bacterial overgrowth 3. **Transition zone** — Between small and large bowel; altered microbiota 4. **Immunological factors** — Highest concentration of M cells and lymphoid follicles ### Distinguishing Features: Crohn's vs. Ulcerative Colitis | Feature | Crohn's Disease | Ulcerative Colitis | |---------|-----------------|-------------------| | **Most common site** | Terminal ileum + cecum | Rectosigmoid | | **Pattern** | Skip lesions (discontinuous) | Continuous | | **Depth** | Transmural | Mucosa + submucosa | | **Small bowel** | Frequently involved (40%) | Never | | **Granulomas** | Present (non-caseating) in 30–50% | Absent | | **Fistulas** | Common | Absent | **Clinical Pearl:** Patients with Crohn's disease involving the terminal ileum often present with right lower quadrant pain (mimicking appendicitis) and may develop strictures or fistulas. Vitamin B₁₂ malabsorption is common due to terminal ileum involvement. **Mnemonic:** **CRISP** — Crohn's involves the **R**ectum **I**nfrequently, **S**mall bowel **P**referentially (especially terminal ileum).
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.