This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, Subject name: Anatomy
[{'content': 'Investigation of choice in acute cholecystitis \nSelect and explain. \nA. OCG, B. HIDA scan, C. USG, D. CT', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Investigation of choice for acute cholecystitis : USG Gold standard for diagnosis of acute cholecystitis : HIDA scan USG is investigation for acute calculous cholecystitis, chronic cholecystitis and cholelithiasis Ref: Sabiston 20th edition Pgno :1493', 'role': 'assistant'}]