**Core Concept**
Hyperophic pyloric stenosis (HPS) is a condition characterized by the thickening of the pylorus muscle, leading to gastric outflow obstruction in infants. The pathophysiology involves an imbalance of gastric and intestinal motility, resulting in persistent vomiting and dehydration.

**Why the Correct Answer is Right**
The correct answer states that infants with HPS typically present with projectile vomiting. This is because the thickened pylorus muscle creates a narrow, rigid channel for gastric emptying, leading to rapid, forceful ejection of stomach contents. The presence of projectile vomiting is a classic clinical sign of HPS, often prompting further investigation with ultrasound to confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Delayed gastric emptying is incorrect because HPS is characterized by rapid gastric emptying, which leads to projectile vomiting.
**Option B:** Loss of appetite is incorrect because infants with HPS often have an increased appetite due to the metabolic consequences of persistent vomiting.
**Option C:** Abdominal tenderness is incorrect because HPS typically presents with a soft, non-tender abdomen.

**Clinical Pearl / High-Yield Fact**
The "olive" sign is a classic clinical finding in HPS, where a palpable, mobile mass is felt in the right upper quadrant, representing the hypertrophied pylorus muscle.

**Correct Answer:** D.