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': 'Which is the most common cause of Cushing syndrome \nSelect and explain. \nA. ACTH producing pituitary adenoma, B. Steroid supplementation, C. Ectopic ACTH producing tumor, D. Mccune Albright syndrome', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]