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
Which structure is most dilated? 
Select and explain. 
A. Aorta, B. Left atrium, C. Left ventricle, D. Right atrium
B: Left atrium
Answer: B: Left atrium 
Explanation: 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.