**Core Concept**
Hyperrophic pyloric stenosis (HPS) is a condition characterized by thickening of the pyloric muscle in infants, leading to gastric outlet obstruction. The investigation of choice should ideally be non-invasive, accurate, and reliable for diagnosing HPS.

**Why the Correct Answer is Right**
The correct investigation of choice for diagnosing HPS is ultrasonography. This is because ultrasonography can accurately measure the thickness of the pyloric muscle and the length of the pyloric channel. A pyloric muscle thickness of more than 4 mm and a pyloric channel length of more than 16 mm are considered diagnostic of HPS. Ultrasonography is also non-invasive and can be performed at the bedside, making it a convenient and safe option for infants.

**Why Each Wrong Option is Incorrect**
**Option A:** Upper GI series with barium. While this can show the narrowing of the pyloric channel, it is not as sensitive or specific as ultrasonography for diagnosing HPS.

**Option B:** Endoscopy. Although endoscopy can visualize the pyloric channel, it is an invasive procedure and may not be necessary for diagnosis.

**Option C:** CT scan. CT scans are not typically used for diagnosing HPS in infants due to radiation exposure and the availability of safer alternatives.

**Clinical Pearl / High-Yield Fact**
Ultrasonography is a valuable tool in the diagnosis of HPS, and a pyloric muscle thickness of more than 4 mm is a reliable criterion for diagnosis.

**Correct Answer:** C. Ultrasonography