This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thorax: Aerial supply, Subject name: Anatomy
[{'content': 'Structure lying at marker C ? \nSelect and explain. \nA. Subclan vein, B. Subclan aery, C. Scalenus anterior, D. Scalenus medius', 'role': 'user'}, {'content': "Answer: C \nExplanation: Scalenus anterior muscle Attaches to the scalene tubercle on the inner(medial) border of 1st rib. * It's tendon is sandwiched between subclan vein (passing anterior) and subclan aery (passing posterior). Both vessels cross over the superior surface of 1st rib. Key: A - Area for attachment of scalenus medius B - Groove for subclan aeryC - Scalene tubercle (attachment of anterior scalene) D - Groove for subclan vein", 'role': 'assistant'}]