This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Medicine
About cushing syndrome, true Is - 
Select and explain. 
A. Low dose dexamethasone suppress coisone secretion, B. CA of adrenal is more common than adenoma, C. Pituitary adenoma size > 2 cm (usually), D. ACTH secretion is the commonest cause
D: ACTH secretion is the commonest cause
Answer: D: ACTH secretion is the commonest cause 
Explanation: most cases the cause is bilateral adrenal hyperplasia due to hyper secretion of pituitary ACTH or ectopic production of ACTH by a non pituitary source ( Harrison 17 pg 2255)