The most common site of intestinal obstruction in gallstone ileus is:
**Core Concept**
The question tests knowledge of **gallstone ileus**, a rare cause of **mechanical intestinal obstruction**. Gallstone ileus occurs when a gallstone enters the intestinal tract through a **cholecystoenteric fistula**.
**Why the Correct Answer is Right**
The most common site of obstruction in gallstone ileus is the **ileum**, specifically the terminal ileum, due to its narrower lumen compared to the rest of the small intestine. The stone usually enters the intestine through a fistula between the gallbladder and the **duodenum**, but it often gets stuck in the ileum.
**Why Each Wrong Option is Incorrect**
**Option A:** The duodenum is the usual site of entry for the gallstone into the intestine, not the most common site of obstruction.
**Option C:** The jejunum is also part of the small intestine but is less commonly the site of obstruction compared to the ileum.
**Clinical Pearl / High-Yield Fact**
Gallstone ileus is more common in females and typically occurs in older adults, often with a history of **cholelithiasis**.
**Correct Answer:** D. Terminal ileum.