This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, Subject name: Surgery
The most common site of intestinal abstruction in gallstone ileus is 
Select and explain. 
A. Duodenum, B. Jejunum, C. Ileum, D. Sigmoid colon
C: Ileum
Answer: C: Ileum 
Explanation: .GALLSTONE ILEUS is a type of acute intestinal obstruction, often seen in elderly and is due to blockage by a bolus or mass of gallstones which commonly enter the intestine through cholecystoduodenal fistula or rarely through cholecystointestinal or -gastric fistulas .Gallstones in the gallbladder | Cholecystitis | Suppuration and adhesion over the duodenal wall | Communication of gallbladder into the duodenum | Gallstones pass into the duodenum forms a bolus ('Rolling stone gather mass') | Blocks narrow pa in the ileum. | Gallstone ileus ref:SRB&;S manual of surgery,ed 3,pg no 594