This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S., Subject name: Medicine
[{'content': 'Which structure is most dilated? \nSelect and explain. \nA. Aorta, B. Left atrium, C. Left ventricle, D. Right atrium', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Answer B. Left atriumThe chest radiograph reveals cardiomegaly, with splaying of the carina and an elevated left main bronchus. These findings are most suggestive of an enlarged left atrium.', 'role': 'assistant'}]