**Core Concept**
Congenital hypertrophic pyloric stenosis (CHPS) is a condition characterized by thickening of the pyloric muscle, leading to gastric outflow obstruction in infants. The abdominal mass in CHPS is a palpable, firm, and non-tender mass located in the epigastric region.

**Why the Correct Answer is Right**
The mass in CHPS is best palpated in the **epigastric region**, just below the xiphoid process, due to the anatomical location of the pylorus. The pylorus is a part of the stomach that connects it to the small intestine, and in CHPS, the pyloric muscle thickens, causing the pylorus to become a palpable mass.

**Why Each Wrong Option is Incorrect**
**Option A:** The left upper quadrant is more likely to be associated with splenomegaly or other abdominal pathologies in infants.
**Option B:** The right upper quadrant is more commonly associated with gallbladder pathology or liver disease.
**Option C:** The right lower quadrant is associated with appendicitis or other right lower quadrant pathologies.

**Clinical Pearl / High-Yield Fact**
In CHPS, the mass is usually best palpable when the infant is in the **supine position**, as this allows the pylorus to be pushed against the abdominal wall, making it more palpable.

**Correct Answer: D. Epigastric region.**