A 72-year-old woman presents with progressive abdominal distension, constipation, and abdominal pain. CT abdomen shows markedly dilated colon with a transition zone at the rectosigmoid junction. What is the most common site of large bowel obstruction?

Explanation

## Most Common Site of Large Bowel Obstruction **Key Point:** The rectosigmoid junction is the most common site of large bowel obstruction, accounting for 50–60% of all colonic obstructions. ### Anatomical Sites of Large Bowel Obstruction | Site | Frequency | Common Causes | Anatomical Basis | | --- | --- | --- | --- | | Rectosigmoid | 50–60% | Colorectal cancer, diverticulitis, volvulus | Narrowest part of colon; highest intraluminal pressure | | Splenic flexure | 10–15% | Carcinoma, volvulus | Acute angle; potential fixation point | | Hepatic flexure | 5–10% | Carcinoma, adhesions | Less common obstruction site | | Cecum | 10–15% | Carcinoma, ileocecal intussusception | Widest diameter; competent ileocecal valve | | Sigmoid colon | 20–30% | Sigmoid volvulus, diverticulitis, cancer | Overlaps with rectosigmoid; high-risk segment | **High-Yield:** The rectosigmoid region combines the narrowest luminal diameter with the highest intraluminal pressure, making it the "point of no return" for obstructing lesions. ### Why the Rectosigmoid Is Most Vulnerable 1. **Anatomical factors:** - Smallest luminal diameter of the entire colon - Taeniae coli create a narrowed lumen - Highest intraluminal pressures (Laplace's law) - Retroperitoneal fixation limits distensibility 2. **Pathological factors:** - Colorectal cancer preferentially arises in the left colon and rectum - Diverticular disease is common in the sigmoid - Sigmoid volvulus occurs at the rectosigmoid junction - Strictures from inflammatory bowel disease or ischemia localize here ### Imaging Features **Plain radiograph:** - Dilated colon proximal to rectosigmoid - Transition zone at rectosigmoid junction - "Bird's beak" appearance if volvulus - Haustra may be preserved proximal to obstruction **CT abdomen (gold standard):** - Abrupt change in caliber at rectosigmoid - Focal narrowing with shouldering (suggests malignancy) - Proximal bowel dilatation - Assessment of cause (mass, stricture, volvulus) **Clinical Pearl:** In elderly patients with acute large bowel obstruction and a rectosigmoid transition zone, colorectal cancer must be excluded until proven otherwise. Colonoscopy or contrast enema is often needed for definitive diagnosis. **Warning:** Do not confuse rectosigmoid obstruction with small bowel obstruction. Large bowel obstruction typically presents with constipation and distension rather than vomiting, and the transition zone is more distal. **Mnemonic:** **FLEXES** — Sites of large bowel obstruction (in order of frequency): Flexures (splenic > hepatic), Left colon/rectosigmoid (most common), Ileocecal junction, Cecum, Sigmoid, Splenic. The rectosigmoid dominates the "left colon" category.