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: Anatomy
[{'content': 'The commonest site of obstruction in Gallstone ileus is - \nSelect and explain. \nA. Proximal ileum, B. Distal ileum, C. Ileocecal junction, D. Transverse colon', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Although Harrison writes the most common site to be the ileocecal valve; Sabiston, Bailey & Maingot\'s write \'distal ileum\' to be the most common site. "The site of obstruction is usually the terminal ileum, just proximal to the ileocecal valve. This corresponds to the segment of the intestine with the smallest diameter." Bailey & Love write, "classically there is impaction about 60 cm proximal to the ileocecal valve." Also, know the commonest site for gallstone impaction in CBD - Ampulla of Vater. Ref : Maingot\'s 10/e, p 1736', 'role': 'assistant'}]