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
Earliest sign of left atrial enlargement is – 
Select and explain. 
A. Posterior displacement of esophagus, B. Widening of carinal angle, C. Elevation of left bronchus, D. Double shadow of right border
A: Posterior displacement of esophagus
Answer: A: Posterior displacement of esophagus 
Explanation: The earliest signs of left atrial enlargement are straightening of the upper left border of the cardiac silhouette and posterior displacement of the esophagus.

The earliest sign of left atrial enlargement on PA view      → Straightening of left heart border.
The earliest sign of left atrial enlargement on lateral view  → Posterior displacement of the esophagus.
The overall earliest sign of left atrial enlargement             → Straightening of left heart border on PA view.