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
The most impoant sign of significance of renal aery stenosis on an angiogram is: 
Select and explain. 
A. 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
B: Presence of collaterals
Answer: B: Presence of collaterals 
Explanation: B i.e. Presence of collaterals