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
MRI is the Investigation of choice for which of the following? 
Select and explain. 
A. Renal stones, B. Renal aery stenosis, C. High grade VUR, D. Renal transplant rejection
B: Renal aery stenosis
Answer: B: Renal aery stenosis 
Explanation: Contrast-enhanced MRA is sufficiently sensitive to be used as a screening test for stenosis of the main renal aery. MRA has lower spatial resolution compared with conventional film-based angiography, and therefore the detection of small-vessel abnormalities, such as vasculitis and distal vasospasm, is problematic.