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What is Earliest sign of left atrial enlargement is – and explain in detail?
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.