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
[{'content': 'Which pa of subclan aery is affected by stenosis is: \nSelect and explain. \nA. First pa, B. Second pa, C. Third pa, D. All pas are equally affected', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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', 'role': 'assistant'}]