**Core Concept**
Left atrial enlargement is a common consequence of various cardiovascular diseases, leading to an increase in the left atrial volume. This enlargement can be detected through various diagnostic methods, including electrocardiography (ECG), chest X-rays, and echocardiography.

**Why the Correct Answer is Right**
The earliest sign of left atrial enlargement is an increase in the P wave amplitude in lead II of the ECG, representing the enlarged left atrium's contribution to the P wave. This is due to the increased volume of the left atrium, which prolongs and widens the P wave. The P wave amplitude in lead II is a sensitive marker for left atrial enlargement, often detectable before other signs become apparent.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because left ventricular hypertrophy is a sign of left ventricular enlargement, not left atrial enlargement.
**Option B:** This option is incorrect because a third heart sound (S3) is a sign of increased left ventricular volume and/or rapid filling, not left atrial enlargement.
**Option C:** This option is incorrect because a widened QRS complex is a sign of left ventricular hypertrophy or bundle branch block, not left atrial enlargement.

**Clinical Pearl / High-Yield Fact**
The P wave amplitude in lead II is a key indicator of left atrial enlargement, and its measurement is often used as a screening tool in patients with suspected cardiac disease.

**Correct Answer:** A. Increased P wave amplitude in lead II.