This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
[{'content': "Confirmatory test for cushing's disease is \nSelect and explain. \nA. High dose dexamethasone suppression test, B. 24 hr urine free cortisol, C. Overnight dexamethasone suppression test, D. Bilateral inferior petrous sinus sampling", 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]