About Cushing syndrome, true is:

Answer is D (Increased ACTH secretion is the commonest endogenous cause): The most common endogenous cause of Cushing's syndrome is increased ACTH secretion by a pituitary adenoma. In Cushing syndrome low dose dexamethasone fails to suppress coisol secretion 'In normal subjects administration of supraphpiological doses of glucocoicoid results in suppression of ACTH and hence of coisol secretion. In Cushing's syndrome of whatever cause there is failure of this suppression when low doses of synthetic glucocoicoid dexamethasone are given'. - Oxford Textbook of Medicine 4th/246 Carcinoma of Adrenal is not more common than Adrenal Adenoma as a cause for Cushing's syndrome 'Adrenal Adenoma are responsible for about the same number of cases as Adrenal Carcinomas'. - Oxford Textbook 'Adrenal Neoplasms are usually unilateral and about half are malignant' - Harrison The size of pituitary Adenoma is usually < 1 cm and not > 2 cm Most common cause of Cushing's syndrome (endogenous) is a Pituitary microadenoma which is less than 10 mm in diameter - Harrison