A 32-year-old woman from Bangalore presents with a 2-year history of recurrent abdominal pain, non-bloody diarrhoea, and weight loss. She reports perianal pain and has a visible perianal fistula. Colonoscopy shows patchy areas of inflammation with normal mucosa between lesions, and the terminal ileum is also involved. Biopsy demonstrates non-caseating granulomas and transmural inflammation. What is the most likely diagnosis, and which complication is she at highest risk for?

Explanation

## Diagnostic Features of Crohn Disease **Key Point:** The combination of skip lesions (patchy inflammation with normal mucosa between), terminal ileum involvement, non-caseating granulomas, transmural inflammation, and perianal fistula is pathognomonic for Crohn disease. ## Pathological Hallmarks | Feature | Finding in This Case | Significance | |---------|----------------------|---------------| | Distribution | Skip lesions + terminal ileum | Segmental, transmural pattern | | Granulomas | Non-caseating | Present in 30–50% of Crohn cases; highly specific | | Depth | Transmural | Affects all layers; explains fistulae and strictures | | Perianal involvement | Fistula present | Pathognomonic for Crohn disease | | Stool character | Non-bloody diarrhoea | UC typically presents with bloody diarrhoea | **High-Yield:** Perianal disease (fistulae, skin tags, abscesses) is virtually diagnostic of Crohn disease and occurs in 20–30% of patients. ## Transmural Inflammation and Complications **Clinical Pearl:** Transmural inflammation in Crohn disease penetrates all layers of the bowel wall, leading to: 1. **Fistula formation** (enterocutaneous, enterovesical, enterovaginal) 2. **Abscess formation** — from contained perforation 3. **Strictures** — from fibrosis and scarring 4. **Free perforation** — less common than in UC because transmural inflammation is often walled off **Mnemonic:** Crohn's **T**ransmural complications: **T**unnelling (fistulae), **T**hickening (strictures), **T**ypical of Crohn's. ## Why Enterocutaneous Fistula Is the Greatest Risk The presence of a visible perianal fistula indicates active transmural disease. This patient is at highest risk for: - Expansion of existing fistula - Formation of new fistulae (enterocutaneous, enterovesical) - Abscess formation from contained perforation - Sepsis from fistula-related infection **Warning:** Do not confuse fistula risk with cancer risk. While Crohn disease increases colorectal cancer risk (1.5–2× baseline), this is a long-term complication, not the immediate risk in active disease. ## Why UC Complications Are Excluded - **Toxic megacolon:** Occurs in UC (and rarely severe Crohn colitis), not in this segmental small bowel–predominant disease. - **Perforation:** More common in UC (mucosal disease); Crohn transmural inflammation is usually walled off by adjacent tissues, preventing free perforation. - **Colorectal cancer:** A late complication (15–20 years), not the acute/subacute risk in active disease.