A 62-year-old man presents with acute abdominal distension, colicky pain, and vomiting. Plain abdominal radiograph shows dilated small bowel loops with air-fluid levels. Regarding the imaging features of small bowel obstruction, all of the following are characteristic findings EXCEPT:

Explanation

## Imaging Features of Small Bowel Obstruction ### Characteristic Plain Film Findings **Key Point:** Rigler's triad (pneumoperitoneum, small bowel obstruction pattern, and ectopic gas in biliary tree) is seen only in gallstone ileus and is NOT present in all cases of small bowel obstruction. | Feature | Characteristic | Frequency | |---------|---|---| | Valvulae conniventes | Traverse entire lumen width (unlike haustra in colon) | Present in SBO | | Transition zone | Abrupt change from dilated to collapsed bowel | Typical finding | | Rigler's triad | Pneumoperitoneum + SBO pattern + pneumobilia | Only in gallstone ileus (~10% of SBO) | | Stacked coin appearance | Concertina-like configuration in closed-loop obstruction | Seen in volvulus, adhesions | ### Why Each Option Is Correct (Except One) **Option 1 — Valvulae conniventes:** True. These plicae circulares are a hallmark of small bowel and traverse the entire width of the lumen, distinguishing small bowel from colon (which has haustra that don't cross the full width). **Option 2 — Transition zone:** True. The abrupt change from proximal dilated bowel to distal collapsed bowel is a cardinal imaging sign of mechanical obstruction and helps localize the site of obstruction. **Option 3 — Rigler's triad:** FALSE. Rigler's triad is specific to gallstone ileus (pneumoperitoneum from a cholecystoenteric fistula, small bowel obstruction pattern, and pneumobilia from the fistula). It is NOT present in all cases of small bowel obstruction—in fact, it is rare (seen in only ~10% of gallstone ileus cases and absent in other causes of SBO). **Option 4 — Stacked coin appearance:** True. This concertina or accordion-like configuration is characteristic of closed-loop obstruction, particularly in volvulus and some adhesive bands, and represents loops of bowel folded upon themselves. **High-Yield:** Rigler's triad is a classic teaching point but is pathognomonic only for gallstone ileus, not for small bowel obstruction in general. Students often confuse "classic" with "always present." **Clinical Pearl:** CT with IV contrast is now the imaging modality of choice for suspected small bowel obstruction, as it can identify the transition zone, assess for strangulation (bowel wall enhancement, mesenteric edema), and determine the underlying cause (adhesions, hernia, malignancy, volvulus).