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
Most common site for impaction of gell stones in gall stone lieus is : 
Select and explain. 
A. Ist pa of duodenum, B. IInd pa of duodenum, C. Terminal ileum, D. Colon
C: Terminal ileum
Answer: C: Terminal ileum 
Explanation: 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.