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
Most common cause of ectopic ACTH production 
Select and explain. 
A. Renal cell carcinoma, B. Hepatocellular carcinoma, C. Small cell carcinoma lung, D. Pheochromacytoma
C: Small cell carcinoma lung
Answer: C: Small cell carcinoma lung 
Explanation: None