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
[{'content': 'About cushing syndrome, true Is - \nSelect and explain. \nA. 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', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 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)', 'role': 'assistant'}]