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': 'The most impoant sign of significance of renal aery stenosis on an angiogram is: \nSelect and explain. \nA. A percentage diameter stenosis >70%, B. Presence of collaterals, C. A systolic pressure gradient >20 mmHg across the lesion, D. Post stenotic dilatation of the renal aery', 'role': 'user'}, {'content': 'Answer: B \nExplanation: B i.e. Presence of collaterals', 'role': 'assistant'}]