**Core Concept**
Neonatal hypertrophic pyloric stenosis (NHPS) is a condition characterized by the thickening of the pyloric muscle, leading to gastric outlet obstruction. The diagnosis of NHPS is crucial for timely intervention to prevent complications such as dehydration and electrolyte imbalances.

**Why the Correct Answer is Right**
Ultrasound is the diagnostic imaging modality of choice for NHPS due to its high sensitivity and specificity. The ultrasound examination typically involves scanning the upper abdomen to visualize the pyloric canal and assess its thickness. A pyloric thickness of greater than 4 mm is considered abnormal. The ultrasound can also evaluate the pyloric muscle layer and assess for any other abnormalities. The use of ultrasound has become the standard diagnostic approach for NHPS due to its non-invasive nature, lack of radiation exposure, and ability to provide real-time images.

**Why Each Wrong Option is Incorrect**
**Option A:** X Ray - While X-rays can provide information on the upper gastrointestinal tract, they are not sensitive enough to diagnose NHPS. X-rays may show signs of gastric distension, but they cannot accurately assess the thickness of the pyloric muscle.

**Option B:** CT scan - CT scans are not typically used for diagnosing NHPS due to radiation exposure and the lack of specificity. CT scans can provide detailed images of the upper abdomen, but they are not necessary for diagnosing NHPS.

**Option C:** MRI - Magnetic resonance imaging (MRI) is not typically used for diagnosing NHPS due to its high cost, limited availability, and radiation exposure. MRI can provide detailed images of the upper abdomen, but it is not necessary for diagnosing NHPS.

**Clinical Pearl / High-Yield Fact**
A key clinical finding in NHPS is the presence of a palpable "olive" mass in the right upper quadrant, which is actually the hypertrophied pyloric muscle. This finding is often associated with the condition.

**✓ Correct Answer: D. Ultrasound. Ultrasound is the diagnostic imaging modality of choice for neonatal hypertrophic pyloric stenosis.**