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
Confirmatory test for cushing's disease is 
Select and explain. 
A. High dose dexamethasone suppression test, B. 24 hr urine free cortisol, C. Overnight dexamethasone suppression test, D. Bilateral inferior petrous sinus sampling
D: Bilateral inferior petrous sinus sampling
Answer: D: Bilateral inferior petrous sinus sampling 
Explanation: None