<prompt> Answer this question truthfullyWhat is the most likely cause of ACTH-dependent Cushing syndrome in a patient who shows no cortisol suppression following a high-dose dexamethasone suppression test 8mg?  <response>: <reason> If a patient with ACTH-dependent Cushing syndrome shows no cortisol suppression following a high-dose dexamethasone suppression test 8mg, the most likely cause of their syndrome is ectopic ACTH secretion. 