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': 'An elderly man presents with a history of jaundice and pain abdomen. The investigation of choice is: \nSelect and explain. \nA. Ultrasound, B. 4-quadrant aspiration, C. CT scan, D. X-ray abdomen', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Ultrasound', 'role': 'assistant'}]