**Core Concept**
Pathological gastroesophageal reflux in infants, also known as gastroesophageal reflux disease (GERD), is characterized by the backflow of stomach contents into the esophagus, leading to symptoms such as regurgitation, vomiting, and difficulty feeding. The earliest indicator of GERD in infants is crucial for early diagnosis and management.

**Why the Correct Answer is Right**
The correct answer is **A. Regurgitation**. Regurgitation is the earliest and most common symptom of GERD in infants. It occurs due to the incompetence of the lower esophageal sphincter (LES) and the transient relaxation of the LES, allowing stomach contents to flow back into the esophagus. Regurgitation is often followed by vomiting, which can be projectile.

**Why Each Wrong Option is Incorrect**
**Option B:** **Vomiting** is a symptom of GERD, but it is not the earliest indicator. Vomiting typically occurs after regurgitation and is often projectile in nature.

**Option C:** **Abdominal distension** can be a symptom of GERD, but it is not the earliest indicator. Abdominal distension can also be caused by other conditions, such as constipation or gas.

**Clinical Pearl / High-Yield Fact**
Remember the "Rule of Three" for GERD in infants: Regurgitation occurs within 30 minutes of feeding, Vomiting is projectile and occurs after regurgitation, and Abdominal distension is a late sign.

**Correct Answer:** A. Regurgitation