**Core Concept**
Neonatal hypertrophic pyloric stenosis (NHPS) is a condition characterized by thickening of the pyloric muscle, leading to gastric outlet obstruction. The investigation of choice should ideally be non-invasive, easily accessible, and able to provide a clear diagnosis.

**Why the Correct Answer is Right**
The investigation of choice for NHPS is ultrasound, which is a non-invasive imaging modality that allows for real-time visualization of the pyloric muscle thickness and length. This is particularly useful in infants, where radiation exposure should be minimized. Ultrasound has a high sensitivity and specificity for diagnosing NHPS, with a reported accuracy of up to 98%.

**Why Each Wrong Option is Incorrect**
**Option A:** **Barium swallow** is not the investigation of choice for NHPS, as it may show a "string sign" or "bird's beak" appearance, but it is not as sensitive or specific as ultrasound.
**Option B:** **Gastrografin enema** is not typically used for diagnosing NHPS, as it involves radiation exposure and may not provide a clear diagnosis.
**Option C:** **Endoscopy** is not usually necessary for diagnosing NHPS, as it is a relatively rare condition and ultrasound is sufficient for diagnosis.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of NHPS is the presence of projectile vomiting, which is often described as forceful and effortless. In addition, a palpable "olive" mass may be felt in the right upper quadrant, which is a classic physical examination finding.

**Correct Answer:** C. Ultrasound.