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: Radiology
[{'content': 'Best investigation for diagnosis of ampullary gall stone with obstructive jaundice is: \nSelect and explain. \nA. Intravenous cholecystography, B. OCG, C. PTC, D. ERCP', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. ERCP', 'role': 'assistant'}]