## **Core Concept**
Hypertrophic pyloric stenosis is a condition characterized by the thickening of the pyloric muscle, leading to gastric outlet obstruction. This condition primarily affects infants and is a common cause of vomiting in this age group. The obstruction results from the narrowing of the pyloric canal due to muscle hypertrophy.

## **Why the Correct Answer is Right**
The correct answer, **C. projectile vomiting**, is a classic symptom of hypertrophic pyloric stenosis. This condition leads to a gastric outlet obstruction, causing the infant to vomit forcefully, often in a projectile manner. The vomiting is typically non-bilious because the obstruction is proximal to the ampulla of Vater, where bile enters the intestine.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while abdominal mass (the "olive") might be palpable in some cases of hypertrophic pyloric stenosis, it is not the most common or direct consequence of the condition.
- **Option B:** This option is incorrect as gastric perforation is not a typical presentation of hypertrophic pyloric stenosis. The condition leads to obstruction, not perforation.
- **Option D:** This option is incorrect because metabolic acidosis is not a direct result of hypertrophic pyloric stenosis. Instead, the condition often leads to metabolic alkalosis due to the loss of gastric acid through vomiting.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the diagnosis of hypertrophic pyloric stenosis can often be made clinically, based on the history of projectile vomiting in an infant and the presence of a palpable "olive" mass in the epigastrium. Ultrasound confirmation is often sought and shows a thickened pyloric muscle and a narrow pyloric canal.

## **Correct Answer: C. projectile vomiting**