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: Radiology
[{'content': 'Earliest sign of left atrial enlargement is – \nSelect and explain. \nA. Posterior displacement of esophagus, B. Widening of carinal angle, C. Elevation of left bronchus, D. Double shadow of right border', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The earliest signs of left atrial enlargement are straightening of the upper left border of the cardiac silhouette and posterior displacement of the esophagus.\n\nThe earliest sign of left atrial enlargement on PA view \xa0 \xa0 \xa0→\xa0Straightening of left heart border.\nThe earliest sign of left atrial enlargement on lateral view \xa0→\xa0Posterior displacement of\xa0the esophagus.\nThe overall earliest sign of left atrial enlargement\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0→\xa0Straightening of left heart border on PA view.', 'role': 'assistant'}]