A 54-year-old female patient presents with abdominal pain and bilious vomiting. She had a history of abdominal hysterectomy 2 years back. On examination, abdomen auscultation revealed high-pitched bowel sounds. Investigations showed air within the biliary tree. What is the most likely diagnosis?

Correct Answer: A. Gallstone ileus

Gallstone ileus is a mechanical bowel obstruction caused by an impacted gallstone in the intestinal lumen, typically at the ileocecal valve. The clinical triad here is pathognomonic: (1) pneumobilia (air in the biliary tree), (2) mechanical small bowel obstruction (high-pitched bowel sounds, bilious vomiting, abdominal pain), and (3) history of biliary disease (implied by the stone). Pneumobilia occurs when a large gallstone erodes through the gallbladder wall into the adjacent bowel (usually duodenum), creating a cholecystoenteric fistula. The stone then travels distally and impacts at the narrowest part of the small bowel—the ileocecal valve—causing obstruction. This accounts for 1–4% of all intestinal obstructions in the West but up to 25% in elderly patients without prior abdominal surgery. The prior hysterectomy is a red herring; while it increases adhesion risk, the presence of pneumobilia is the discriminating finding that rules out simple adhesions. On imaging (CT/X-ray), Rigler's triad is diagnostic: pneumobilia, small bowel obstruction, and an ectopic gallstone visible on imaging. Management is enterotomy with stone extraction; cholecystectomy and fistula repair are deferred unless acute cholecystitis is present. In Indian practice, this diagnosis is often missed in elderly women presenting with recurrent obstruction.

Why the other options are wrong

B. Diverticulitis — Diverticulitis presents with left lower quadrant pain and fever, typically in older patients with sigmoid involvement. It does NOT cause pneumobilia or mechanical small bowel obstruction with high-pitched bowel sounds. Imaging shows colonic wall thickening and pericolic fat stranding, not air in the biliary tree. This is a trap for students who see 'abdominal pain' without reading the pneumobilia clue. C. Adhesive intestinal obstruction — While the hysterectomy history makes adhesions likely, adhesions cause mechanical obstruction WITHOUT pneumobilia. Pneumobilia is the discriminating finding here—it requires a fistula between the biliary system and bowel, which adhesions alone cannot create. Students may anchor on 'prior surgery' and miss the pathognomonic imaging finding. D. Ischemic enterocolitis — Ischemic bowel presents with severe pain out of proportion to examination, bloody diarrhea, and systemic toxicity. It does NOT produce pneumobilia or mechanical obstruction with high-pitched sounds. Imaging shows bowel wall thinning, thumbprinting, or necrosis—not air in the biliary tree. This is a distractor for students confusing obstruction with ischemia.

High-Yield Facts

Mnemonics

RIGLER'S TRIAD Pneumobilia + Intestinal obstruction + Radiopaque stone = Gallstone ileus. Remember: Rigler = Radio-opaque stone in the bowel. PNEUMOBILIA = FISTULA Air in biliary tree means a fistula between bowel and biliary system. Only gallstone ileus (via erosion) and incompetent sphincter of Oddi (rare) cause this. In obstruction + pneumobilia, think gallstone ileus first.

NBE Trap

NBE pairs "prior abdominal surgery" with "mechanical obstruction" to lure students toward adhesions, but the presence of pneumobilia is the discriminating finding that only gallstone ileus explains. Students who anchor on surgical history without reading imaging findings will miss this.

Clinical Pearl

In Indian elderly women presenting with recurrent small bowel obstruction and no prior abdominal surgery, always check for pneumobilia—gallstone ileus is often missed because it is rare in younger populations. A single CT abdomen with pneumobilia + obstruction pattern can save the patient from unnecessary adhesiolysis.

_Reference: Bailey & Love Ch. 65 (Intestinal Obstruction); Robbins Ch. 17 (Biliary Tract)_