A 28-year-old woman from Bangalore with a 3-year history of bloody diarrhea and tenesmus undergoes colonoscopy. The mucosa shows continuous inflammation limited to the colon, with friability and ulceration but no skip lesions. Biopsies reveal crypt abscesses and crypt distortion. Which investigation would best differentiate this condition from Crohn disease and confirm the diagnosis?

Explanation

## Histopathological Differentiation of Ulcerative Colitis from Crohn Disease **Key Point:** Rectal biopsy histopathology is the most specific investigation to confirm ulcerative colitis (UC) and differentiate it from Crohn disease. The hallmark features are crypt abscesses, crypt distortion, and mucin depletion in goblet cells — all limited to the mucosa and submucosa. ### Pathological Features Diagnostic of UC **High-Yield:** The microscopic features that confirm UC and exclude Crohn disease: | Feature | Ulcerative Colitis | Crohn Disease | |---|---|---| | **Depth of inflammation** | Mucosa and submucosa only | Transmural (full thickness) | | **Crypt abscesses** | Present (hallmark) | Rare | | **Crypt distortion** | Marked | Mild | | **Goblet cell mucin** | Depleted | Normal | | **Distribution** | Continuous, starts at rectum | Patchy (skip lesions) | | **Granulomas** | Absent | Present in 50% | | **Small bowel involvement** | Never | Common (~75%) | **Clinical Pearl:** Crypt abscesses (neutrophils within crypt epithelium) are virtually pathognomonic for UC and rarely seen in Crohn disease. Combined with mucin depletion and absence of transmural inflammation, they confirm the diagnosis. ### Why Histopathology is the Gold Standard In this case, colonoscopy findings (continuous colonic inflammation, friability, ulceration) are suggestive of UC, but histopathology is required to: 1. Confirm mucosal-only inflammation (excluding transmural Crohn disease) 2. Identify crypt abscesses (diagnostic hallmark) 3. Assess for granulomas (present in Crohn, absent in UC) 4. Evaluate mucin depletion in goblet cells (specific for UC) **Mnemonic:** **MUCUS** — **M**ucosal-only inflammation, **U**lcerative pattern, **C**rypt abscesses, **U**niform continuous disease, **S**tarts at rectum (features of UC) ### Why Other Investigations Are Insufficient **Warning:** Colonoscopic findings alone (continuous inflammation, friability) can mimic UC but cannot definitively exclude Crohn disease without histology. Some Crohn patients present with colonic-predominant disease. **Clinical Pearl:** The absence of skip lesions on colonoscopy in this patient strongly suggests UC, but transmural inflammation (diagnostic of Crohn) can only be confirmed by biopsy depth assessment — hence rectal biopsy is essential.