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: Radiology
[{'content': 'Most important sign in hemodynamically significant renal artery stenosis \nSelect and explain. \nA. Presence of collateral vessels, B. Elevated renin level in I/L renal vein  ≥  1.5 : 1, C. Transstenotic pressure gradient  ≥ 40 mm Hg, D. Decrease in renal size', 'role': 'user'}, {'content': 'Answer: A \nExplanation: All are signs of renal artery stenosis.\nMost\xa0 important - Presence\xa0of collateral vessels.', 'role': 'assistant'}]