During a pathology teaching round, a specimen of colon from a patient with inflammatory bowel disease is being reviewed. The histology shows continuous mucosal and submucosal inflammation limited to the mucosa and submucosa, with crypt distortion and crypt abscess formation, but no granulomas. Which of the following is the most common site of disease onset in this patient's condition?

Explanation

## Site of Onset in Ulcerative Colitis **Key Point:** Ulcerative colitis characteristically begins in the rectum and extends proximally in a continuous, uninterrupted fashion. The rectum is involved in >95% of UC cases, making it the most common site of disease onset. ### Anatomical Pattern of Ulcerative Colitis | Pattern | Frequency | Characteristics | |---------|-----------|------------------| | Rectum with proximal extension (continuous) | >95% | Disease always involves rectum; extends proximally | | Proctosigmoiditis | 40–50% | Rectum + sigmoid only | | Left-sided colitis | 30–40% | Rectum to splenic flexure | | Pancolitis | 15–20% | Entire colon involved | | Backwash ileitis | 10–15% | Reflux into terminal ileum (UC, not Crohn) | **High-Yield:** The **rectum is ALWAYS involved in ulcerative colitis** — if the rectum is spared, the diagnosis is NOT UC. This is a cardinal distinguishing feature from Crohn disease. ### Histopathological Features Confirming UC 1. **Mucosal and submucosal involvement only** — does NOT extend into muscularis propria (unlike Crohn) 2. **Continuous inflammation** — no skip lesions; if inflammation stops, it does not resume distally 3. **Crypt distortion and crypt abscess** — characteristic of UC 4. **Absence of granulomas** — present in only 2–5% of UC (vs. 30–50% in Crohn) 5. **Goblet cell depletion** — loss of mucin-secreting cells **Clinical Pearl:** The **rectosigmoid region** bears the brunt of inflammation because it is the most distal and has the longest contact time with fecal contents and luminal antigens. ### Why Continuous Proximal Extension? - **Anatomical continuity** — inflammation spreads proximally along the colon in a contiguous manner - **No skip lesions** — if a segment is involved, all distal segments are also involved - **Splenic flexure** — the proximal limit in left-sided colitis (watershed area with reduced blood supply) - **Ileocecal valve** — acts as a barrier; backwash ileitis is rare and does NOT represent true small bowel Crohn disease ### Comparison: UC vs. Crohn at a Glance | Feature | Ulcerative Colitis | Crohn Disease | |---------|-------------------|---------------| | **Site of onset** | Rectum (>95%) | Terminal ileum (40–50%) | | **Pattern** | Continuous, proximal | Discontinuous, skip lesions | | **Depth** | Mucosal/submucosal | Transmural | | **Rectum involvement** | Always | Variable (60–70%) | | **Granulomas** | 2–5% | 30–50% | | **Fistulas** | Absent | Common | **Mnemonic:** **RUMP** — **R**ectum **U**sually involved in **M**ucosal **P**attern (UC) vs. **TWIG** — **T**erminal ileum **W**ith **I**nflammatory **G**ranulomas (Crohn). [cite:Robbins 10e Ch 17]