This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Oncology, 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: Ectopic ACTH can be secreted from original cancer tissues or metastatic lesions of SCLC. ... We repo a case of SCLC with Cushing syndrome due to EAS from the original tumor tissue in a 69-year-old male presenting with severe hypokalemia, metabolic alkalosis, and hypeension without any respiratory symptoms Ref Harrison 20th edition pg 597