A 32-year-old man from Delhi presents with a 6-month history of bloody diarrhea, abdominal pain, and weight loss. Colonoscopy shows continuous mucosal inflammation limited to the colon with superficial ulceration and a granular appearance. Which investigation would be most helpful to differentiate this condition from Crohn disease?

Explanation

## Differentiating Ulcerative Colitis from Crohn Disease **Key Point:** Ileoscopy with terminal ileal biopsy is the gold standard investigation to differentiate ulcerative colitis (UC) from Crohn disease (CD). The terminal ileum is involved in 95% of Crohn disease cases but is spared in ulcerative colitis. ### Why Terminal Ileal Biopsy? Ulcerative colitis is characterized by: - Inflammation limited to the colon and rectum - Continuous, superficial mucosal involvement - **Normal terminal ileum** (pathognomonic feature) Crohn disease typically shows: - Skip lesions (discontinuous inflammation) - Full-thickness involvement - **Terminal ileal involvement** (95% of cases) - Non-caseating granulomas (50–60% of biopsies) ### Diagnostic Algorithm ```mermaid flowchart TD A[Chronic IBD with colonic involvement]:::outcome --> B{Inflammation pattern?}:::decision B -->|Continuous, superficial| C[Likely UC]:::outcome B -->|Skip lesions or patchy| D[Likely CD]:::outcome C --> E[Ileoscopy + terminal ileal biopsy]:::action D --> E E --> F{Terminal ileum involved?}:::decision F -->|Normal| G[Confirm UC]:::outcome F -->|Inflamed/granulomas| H[Confirm CD]:::outcome ``` **High-Yield:** The terminal ileum is the single most discriminatory site. If it is normal on ileoscopy with biopsy confirmation, UC is virtually certain. If it shows inflammation or granulomas, CD is confirmed. **Clinical Pearl:** Even when colonoscopy suggests UC (continuous colonic inflammation), ileoscopy must be performed to exclude CD, as up to 10% of patients initially thought to have UC are later reclassified as CD based on ileal involvement. ### Comparison Table: Key Histopathologic Differences | Feature | Ulcerative Colitis | Crohn Disease | | --- | --- | --- | | **Distribution** | Continuous, rectum to proximal colon | Skip lesions, any part of GI tract | | **Depth** | Mucosa and submucosa only | Full thickness (transmural) | | **Terminal ileum** | Normal (95% of cases) | Involved (95% of cases) | | **Granulomas** | Absent | Non-caseating (50–60%) | | **Crypt distortion** | Marked | Mild to moderate | | **Fissuring ulcers** | Absent | Present | [cite:Robbins 10e Ch 17]