**Core Concept**
Hypertrophic pyloric stenosis is a condition characterized by thickening of the pyloric muscle, leading to gastric outlet obstruction. This condition is most commonly seen in infants, typically presenting with projectile vomiting.

**Why the Correct Answer is Right**
The thickening of the pyloric muscle is due to an increase in smooth muscle mass, which is a result of an abnormal hypertrophic response. This is often caused by an imbalance in the autonomic nervous system, leading to an increase in sympathetic tone. The pyloric channel, which is the area of the pylorus where the muscle is thickest, is particularly affected. As a result, the pylorus becomes narrowed, obstructing the passage of food from the stomach into the small intestine.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastric atony is not typically seen in hypertrophic pyloric stenosis. In fact, the pylorus is often hyperactive, leading to rapid emptying of the stomach.
**Option B:** Duodenal stenosis is a different condition altogether, typically presenting with different symptoms and a different location of obstruction.
**Option C:** Pyloric ulcer is a possible cause of pyloric stenosis, but it is not the most common cause, and the symptoms and presentation would be different.

**Clinical Pearl / High-Yield Fact**
In hypertrophic pyloric stenosis, the key clinical finding is the presence of a palpable pyloric mass, which is often referred to as "olive" in infants.

**Correct Answer:** C. Pyloric stenosis.