**Core Concept**
Hypertrophic pyloric stenosis is a condition characterized by thickening of the pyloric muscle, leading to gastric outlet obstruction. This condition is more common in infants and is often associated with a firm, olive-like mass in the epigastric region.

**Why the Correct Answer is Right**
The thickening of the pyloric muscle in hypertrophic pyloric stenosis is due to an increase in smooth muscle and fibrosis. This leads to a narrowing of the pyloric channel, causing gastric outlet obstruction and projectile non-bilious vomiting. The condition is often diagnosed in infants around 3-4 weeks of age. The pathophysiology involves an imbalance in the growth and differentiation of the pyloric smooth muscle cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Duplication cyst of the pylorus is a rare congenital anomaly that can cause gastric outlet obstruction, but it would not be associated with a thickened pyloric muscle.

**Option B:** Gastric volvulus is a condition where the stomach twists abnormally, causing obstruction and vomiting, but it would not be associated with a firm, olive-like mass in the epigastric region.

**Option C:** Pyloric atresia is a rare congenital anomaly where there is a complete obstruction of the pyloric channel, but it would not be associated with a thickened pyloric muscle.

**Option D:** Gastric carcinoma is a rare condition in infants and would not be associated with a firm, olive-like mass in the epigastric region.

**Clinical Pearl / High-Yield Fact**
Hypertrophic pyloric stenosis is often associated with a "olive" mass in the epigastric region, and the condition is more common in infants who are breastfed. This condition can be diagnosed based on clinical presentation, physical examination, and ultrasound findings.

**Correct Answer:** B. Hypertrophic pyloric stenosis is characterized by thickening of the pyloric muscle due to an increase in smooth muscle and fibrosis, leading to gastric outlet obstruction.