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: Surgery
A gall stone gets impacted most commonly in which pa of common bile duct - 
Select and explain. 
A. Supraduodenal, B. Retroduodenal, C. Ampulla of vater, D. Common hepatic duct
C: Ampulla of vater
Answer: C: Ampulla of vater 
Explanation: Ans. is 'c' i.e. Ampulla of vater