Step 1: Distinguish physiological reflux from pathological GERD in infants. Almost all healthy infants have some physiological gastroesophageal reflux, which presents as effortless spitting up of feeds (regurgitation) without distress and with normal weight gain.
Step 2: Reflux becomes pathological (GERD) when stomach contents, including acid, move repeatedly into the esophagus and up to or out of the mouth, producing troublesome symptoms or complications.
Step 3: The earliest and most frequent symptom that signals the shift from normal spitting up to true GERD is frequent regurgitation and vomiting. A normal-reflux infant only spits up after a meal; once persistent vomiting and regurgitation appear, it is a pathological sign.
Step 4: Other listed features are not the earliest. Respiratory distress (from aspiration or reflux-triggered airway symptoms) is a late manifestation. Upper GI bleed (from reflux esophagitis) and food bolus obstruction (from peptic stricture) are complications of long-standing disease, not early signs.
Conclusion: The earliest symptom that becomes pathological is regurgitation and vomiting. This matches the printed key (option C).