**Core Concept**
The question tests the understanding of pathological gastroesophageal reflux in infants, specifically the earliest indicator of GERD. Gastroesophageal reflux disease (GERD) is a condition where stomach acid frequently flows back into the tube connecting the mouth and stomach (esophagus).

**Why the Correct Answer is Right**
Although the specific correct answer choice is not provided, the earliest indicator of pathological gastroesophageal reflux in infants often involves symptoms that can be subtle and easily missed. Typically, the earliest signs may include frequent regurgitation or vomiting, but more specific indicators such as Sandifer's syndrome (a condition characterized by torticollis and dystonic body movements in response to reflux episodes) or feeding refusal can also be crucial.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific details of the options, it's challenging to address each directly, but generally, options that do not align with clinical presentations of GERD in infants, such as respiratory symptoms without a clear link to feeding or regurgitation, would be incorrect.
**Option B:** Similarly, this would be incorrect if it does not directly relate to the common or earliest signs of GERD.
**Option C:** And this, if it suggests a later complication or an unrelated symptom.
**Option D:** This option would be incorrect if it represents a symptom or sign that appears later in the disease process or is not specifically associated with GERD.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that while regurgitation is common in infants, pathological GERD is suggested by associations such as feeding difficulties, failure to thrive, or respiratory symptoms.

**Correct Answer:** Correct Answer: D. Sandifer's syndrome.