Which of the following statements about Cushing's disease is true?

Cushing's syndrome is due to increased adrenal coisol production, regardless of aetiology.Cushing's disease usually is caused by a pituitary microadenoma producing high adrenocoicotropin (ACTH) levels.In most patients, coisol levels would be suppressed as a result of the high-dose dexamethasone suppression test.Transsphenoidal microsurgical removal of the microadenoma is the treatment of choice in most centres.