A 72-year-old woman presents with progressive abdominal distension, constipation, and colicky pain. Imaging confirms large bowel obstruction. What is the most common site of large bowel obstruction in India?

Explanation

## Most Common Site of Large Bowel Obstruction ### Anatomical Distribution **Key Point:** The rectosigmoid junction is the most common site of large bowel obstruction, accounting for 40–60% of all colonic obstructions globally and in India. **High-Yield:** The rectosigmoid region is the narrowest and most fixed part of the colon, making it the most vulnerable to obstruction from any cause (malignancy, volvulus, stricture, or fecal impaction). ### Why the Rectosigmoid? 1. **Narrowest luminal diameter** — the colon progressively narrows from caecum to rectum 2. **Fixed anatomy** — the sigmoid is retroperitoneal and less mobile than proximal colon 3. **Increased intraluminal pressure** — Laplace's law predicts higher wall tension in narrower segments 4. **Increased muscular contractions** — taeniae coli create natural constriction points ### Etiology by Site | Site | Most Common Cause | Frequency | |------|-------------------|----------| | **Rectosigmoid** | Colorectal malignancy (60–70%), volvulus (20–30%) | 40–60% of all LBO | | **Splenic flexure** | Malignancy, adhesions | 5–10% | | **Hepatic flexure** | Malignancy, adhesions | 5–10% | | **Caecum** | Malignancy, volvulus | 10–15% | **Clinical Pearl:** In elderly patients presenting with large bowel obstruction, colorectal malignancy at the rectosigmoid junction should be the primary concern until proven otherwise. Volvulus is more common in younger patients or those with chronic constipation. ### Clinical Presentation at Rectosigmoid - Progressive constipation and abdominal distension - Colicky pain (often lower abdomen and left iliac fossa) - Late presentation of fecal vomiting (due to proximal colon distension) - Palpable mass on digital rectal examination (if malignancy) ### Investigation Strategy - **CT colonography** — gold standard for diagnosis and staging - **Flexible sigmoidoscopy** — therapeutic (stent placement) or diagnostic - **Barium enema** — contraindicated if perforation suspected