This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genitourinary radiology, Subject name: Radiology
[{'content': 'The gold standard investigation for renal aery stenosis in an 21 year old girl : \nSelect and explain. \nA. Renal scan, B. Ultrasound Doppler, C. Renal angiography, D. 3D Contrast-enhanced MRA', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Angiography is the gold standard and definitive means to diagnose RAS, except for the risk of contrast reaction. In screening for stenosis of the main renal aeries and differentiation from fibromuscular dysplasia, the use of 3D contrast enhanced MRA provides optimum method for its detection and allows the demonstration of small accessory renal aeries and segmental branches Doppler evaluation has limited application as screening modality. The clinical signs of renal aery stenosis include Renovascular hypeension and azotaemia.', 'role': 'assistant'}]