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
Which is the most impoant sign of significance of renal aery stenosis on an angiogram? 
Select and explain. 
A. Presence of collaterals, B. A percentage diameter stenosis > 70%, C. Post-stenotic dilatation of the renal aery, D. A systolic pressure gradient > 20 mm Hg across the lesion
A: Presence of collaterals
Answer: A: Presence of collaterals 
Explanation: The most impoant sign of significant renal aery stenosis on an angiogram is presence of bridging collaterals. Other signs of hemodynamically significant aerial stenosis are post stenotic dilation, pressure gradient >10mmHg or ipsilateral renal vein renin hypersecretion. Ref: Essentials of Surgery: Scientific Principles and Practice By Lazar J. Greenfield, page 560; Computed Body Tomography With Mri Correlation, Volume 1 By Joseph K. T. Le page 1288