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: Surgery
Which pa of subclan aery is affected by stenosis is: 
Select and explain. 
A. First pa, B. Second pa, C. Third pa, D. All pas are equally affected
A: First pa
Answer: A: First pa 
Explanation: Most common etiology of subclan stenosis is atherosclerosis, and it mostly affects the ostium or proximal pa of subclan aery. Ref: Peripheral and Cerebrovascular Intervention By Deepak L. Bhatt, Page 102