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: Medicine
[{'content': 'Most common cause of ectopic ACTH production \nSelect and explain. \nA. Renal cell carcinoma, B. Hepatocellular carcinoma, C. Small cell carcinoma lung, D. Pheochromacytoma', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]