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': 'The investigation of choice for acute cholecystitis is: \nSelect and explain. \nA. USG, B. HIDA-Scan, C. CT-Scan, D. OCG', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. USG', 'role': 'assistant'}]