**Core Concept**
Congenital Hypertrophic Pyloric Stenosis (CHPS) is a condition characterized by the thickening of the pyloric muscle, leading to gastric outlet obstruction in infants. The clinical presentation typically includes projectile vomiting, weight loss, and a palpable pyloric mass.

**Why the Correct Answer is Right**
Ultrasound (USG) is the preferred diagnostic modality for CHPS due to its non-invasive nature, high sensitivity, and specificity. It allows for the direct visualization of the pyloric muscle thickness, pyloric channel length, and the presence of a pyloric mass. A pyloric muscle thickness of more than 4 mm is considered diagnostic of CHPS. USG is also useful for monitoring the response to treatment and for detecting complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Barium meal is less sensitive and specific for CHPS compared to USG and may not be as readily available. It is also associated with radiation exposure.

**Option C:** CT scan is not recommended for the diagnosis of CHPS due to radiation exposure and the high cost.

**Option D:** MRI is not typically used for the diagnosis of CHPS due to its high cost, limited availability, and the fact that it is not as sensitive as USG for detecting pyloric muscle thickness.

**Clinical Pearl / High-Yield Fact**
In infants with suspected CHPS, a pyloric muscle thickness of more than 4 mm on USG is diagnostic, and a thickness of 3-4 mm is considered suggestive.

**Correct Answer:**
✓ Correct Answer: B. USG