This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Head and neck, Subject name: Anatomy
[{'content': 'Subclan steal syndrome results due to obstruction of \nSelect and explain. \nA. 1st pa of subclan aery, B. 2nd pa of subclan aery, C. 3rd pa of subclan aery, D. Subclan vein', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The proximal pa of left subclan is blocked on left side so no flow in veebral and to left arm. Blood from right veebral enters left veebral and flows back to supply left arm.', 'role': 'assistant'}]