A 28-year-old woman from Delhi presents with a 6-month history of bloody diarrhoea, tenesmus, and left lower abdominal cramping. On colonoscopy, continuous mucosal inflammation with friability and ulceration is seen starting from the rectum and extending proximally to the sigmoid colon. Biopsy shows crypt distortion, crypt abscess formation, and inflammation confined to the mucosa and submucosa. She has no perianal disease or fistulas. What is the most likely diagnosis?

Explanation

## Diagnosis: Ulcerative Colitis ### Key Clinical and Pathological Features **Key Point:** The continuous pattern of inflammation starting from the rectum and extending proximally, combined with mucosal-and-submucosal involvement only, is pathognomonic for ulcerative colitis (UC). ### Distinguishing Histopathology | Feature | Ulcerative Colitis | Crohn Disease | |---------|-------------------|---------------| | **Distribution** | Continuous, rectum to proximal colon | Skip lesions (patchy, discontinuous) | | **Depth of inflammation** | Mucosa + submucosa only | Transmural (all layers) | | **Crypt changes** | Crypt distortion, crypt abscess | Crypt distortion, granulomas (50–60%) | | **Perianal disease** | Absent | Present in 30% | | **Fistulas** | Never | Common | | **Cobblestone appearance** | Absent | Present (due to transmural inflammation) | **High-Yield:** The **absence of perianal disease and fistulas** rules out Crohn disease. The **continuous mucosal inflammation from rectum** rules out skip lesions (Crohn). The **mucosal-and-submucosal confinement** (not transmural) is diagnostic of UC. ### Why This Is NOT Crohn Disease Crohn disease would show: - Skip lesions (patchy areas of normal mucosa between inflamed segments) - Transmural inflammation (involvement of all bowel wall layers) - Granulomas in 50–60% of cases - Perianal complications (fistulas, skin tags, abscesses) in ~30% This patient has **none** of these features. ### Why This Is NOT Infectious Colitis Shigella or other infectious colitis would be: - Acute onset (usually < 2 weeks) - Associated with fever and systemic toxicity - Self-limited (resolves within 4–6 weeks) - Histology shows acute inflammation without crypt distortion or chronic changes This patient has a **6-month chronic course** with architectural crypt changes. ### Why This Is NOT Ischaemic Colitis Ischaemic colitis typically: - Affects the "watershed" areas (splenic flexure, rectosigmoid junction) - Presents acutely with severe pain and bloody diarrhoea - Shows mucosal necrosis and haemorrhage without crypt distortion - Does not cause crypt abscess formation **Clinical Pearl:** UC is the most common form of inflammatory bowel disease in India, and **rectal involvement with continuous proximal extension** is the hallmark. The absence of transmural disease and perianal complications makes Crohn disease unlikely. **Mnemonic — UC vs Crohn:** **"UC = Continuous, Crohn = Cracks (transmural + fistulas)"** - **UC**: Continuous inflammation, **U**lcer in mucosa only, **C**omplications are systemic (arthritis, uveitis) - **Crohn**: **C**racks (fissuring ulcers, transmural), **C**obblestone, **C**omplications are local (fistulas, abscesses)