This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cushing Syndrome, Subject name: Medicine
[{'content': 'Which of the following tests is best for differentiating ACTH secreting pituitary tumour from Ectopic ACTH production? \nSelect and explain. \nA. 24 hour Urinary free coisol, B. Overnight dexamethasone suppression test, C. Low dose dexamethasone suppression test, D. High dose dexamethasone suppression test', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 24 hour urinary coisol levels are elevated in both subsets: ACTH secreting pituitary tumour and Ectopic ACTH secretion. Overnight Dexamethasone suppression test involves overnight 1mg dexamethasone tablet and measurement of 8am morning serum or urinary coisol. It is used to diagnose hypercoisolism but cannot identify etiology of Cushing syndrome. High dose dexamethasone suppression test involves giving 6 hourly dexamethasone 2mg, which leads to etiological diagnosis.( differentiates between ACTH secreting pituitary tumor from Ectopic ACTH ) Serum Coisol> 5mg/dl: Ectopic ACTH production.', 'role': 'assistant'}]