This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': 'Most common site for impaction of gell stones in gall stone lieus is : \nSelect and explain. \nA. Ist pa of duodenum, B. IInd pa of duodenum, C. Terminal ileum, D. Colon', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Answer is C (Terminal ileum):\'Classically there is obstruction / impaction about 60 cm proximal to ileocaecal valve\'- Bailey\'The site of obstruction by the impacted gall stone is usually at the ileocaecal valve" provided that the more proximal S.I. is of normal calibre. - HarrisonsIf both choices come simultaneously in a question go for 60 cm proximal to ileacoecal valve, as, it has been defined as the narrowest pa of small intestine.', 'role': 'assistant'}]