A 32-year-old man with a 6-year history of ulcerative colitis presents with bloody diarrhoea and abdominal pain. Colonoscopy reveals continuous mucosal inflammation with friability and ulceration limited to the colon. Which is the most common site of involvement in ulcerative colitis?

Explanation

## Most Common Site of Ulcerative Colitis Involvement ### Disease Pattern Ulcerative colitis characteristically involves the colon in a **continuous, proximally-extending pattern** starting from the rectum. The rectum and sigmoid colon are affected in the vast majority of cases. ### Frequency of Site Involvement | Site | Frequency | Pattern | |------|-----------|----------| | Proctitis (rectum only) | 40–50% | Distal disease | | Proctosigmoiditis (rectum + sigmoid) | 30–40% | Distal disease | | Left-sided colitis (up to splenic flexure) | 10–20% | Distal to mid-colon | | Pancolitis (entire colon) | 5–10% | Extensive disease | **Key Point:** Rectal involvement is **universal** in ulcerative colitis; the disease never spares the rectum and extends proximally in a continuous manner. This distinguishes it from Crohn disease, which has a patchy, skip-lesion pattern. ### Clinical Significance **High-Yield:** The **rectosigmoid region** is the most commonly and severely affected site in UC. Disease severity and extent correlate with systemic complications and risk of colorectal cancer. **Clinical Pearl:** A patient with UC who has a normal rectosigmoidoscopy should prompt reconsideration of the diagnosis — rectal involvement is mandatory for UC diagnosis. ### Differential from Crohn Disease - **Ulcerative colitis:** Continuous, starts at rectum, limited to colon - **Crohn disease:** Skip lesions, can involve any part of GI tract (mouth to anus), transmural inflammation [cite:Harrison 21e Ch 295]