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
A common cause of Cushing syndrome is - 
Select and explain. 
A. Cancer producing ectopic ACTH, B. Pituitary adenoma, C. Adrenal tuberculosis, D. None of the above
B: Pituitary adenoma
Answer: B: Pituitary adenoma 
Explanation: 75% cases of Cushing's syndrome is caused by ACTH-producing pituitary adenomas (Cushing's disease); Ectopic ACTH syndrome - 15%; adrenocoical adenomas-5 to 10% and adrenocoical carcinoma-1% and other rare causes-<1%. Reference : page 2314 Harrison's Principles of Internal Medicine 19th edition