**Core Concept**
Hypertrophic pyloric stenosis is a clinical condition characterized by the thickening of the pyloric muscle, leading to gastric outlet obstruction. This condition is most commonly seen in infants and is caused by an abnormality in the muscular layer of the pylorus.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of hypertrophic pyloric stenosis. The condition is caused by an increase in the thickness of the pyloric muscle, which leads to a narrowing of the gastric outlet. This narrowing prevents the normal passage of food from the stomach to the small intestine, resulting in projectile vomiting. The thickening of the pyloric muscle is thought to be caused by an abnormality in the muscular layer of the pylorus, which is rich in smooth muscle cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the pathophysiology of hypertrophic pyloric stenosis. While an increase in pyloric muscle thickness is a key feature of the condition, it is not the primary cause of the gastric outlet obstruction.

**Option B:** This option is incorrect because it is a symptom of hypertrophic pyloric stenosis, rather than a characteristic of the condition. Projectile vomiting is a common symptom of the condition, but it is not a defining feature.

**Option C:** This option is incorrect because it is a potential complication of hypertrophic pyloric stenosis, rather than a characteristic of the condition. Malnutrition is a potential complication of the condition, but it is not a defining feature.

**Option D:** This option is incorrect because it is a potential cause of hypertrophic pyloric stenosis, rather than a characteristic of the condition. Hypertrophic pyloric stenosis is a congenital condition, but it is not caused by an increase in stomach acid production.

**Clinical Pearl / High-Yield Fact**
A key feature of hypertrophic pyloric stenosis is the presence of a palpable "olive" in the right upper quadrant of the abdomen. This is a firm, mobile mass that represents the thickened pyloric muscle.

**Correct Answer:** D.