A 32-year-old man with a 6-year history of ulcerative colitis presents with bloody diarrhea and abdominal cramping. Colonoscopy reveals continuous mucosal inflammation with crypt abscesses. Which is the most common site of involvement in ulcerative colitis?

Explanation

## Site of Involvement in Ulcerative Colitis **Key Point:** Ulcerative colitis characteristically involves the colon in a continuous, non-skipping pattern, always starting at the rectum and extending proximally. ### Distribution Pattern | Feature | Ulcerative Colitis | Crohn's Disease | |---------|-------------------|------------------| | **Most common site** | Rectum and sigmoid colon | Terminal ileum and cecum | | **Pattern** | Continuous, no skip lesions | Discontinuous, skip lesions | | **Depth** | Mucosa and submucosa only | Transmural | | **Extent** | Starts at rectum, extends proximally | Can involve any part of GI tract | ### Frequency of Involvement 1. **Proctitis alone** — 20–30% of cases 2. **Left-sided colitis** (rectum to splenic flexure) — 40–50% of cases 3. **Pancolitis** (entire colon) — 20–30% of cases **Clinical Pearl:** The rectum is ALWAYS involved in ulcerative colitis; if the rectum is spared, Crohn's disease should be suspected. **High-Yield:** Rectal involvement with continuous proximal extension is the pathognomonic distribution pattern that distinguishes UC from Crohn's disease. The rectosigmoid region is the most frequent site of initial and persistent inflammation. [cite:Robbins 10e Ch 17]