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
Which is the most common cause of Cushing syndrome 
Select and explain. 
A. ACTH producing pituitary adenoma, B. Steroid supplementation, C. Ectopic ACTH producing tumor, D. Mccune Albright syndrome
A: ACTH producing pituitary adenoma
Answer: A: ACTH producing pituitary adenoma 
Explanation: None