**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 diagnostic challenge lies in identifying this condition through non-invasive means.

**Why the Correct Answer is Right**
The investigation of choice for diagnosing CHPS is the ultrasound of the upper abdomen, which can accurately measure the thickness of the pyloric muscle and assess the pyloric channel length. This non-invasive imaging modality is both sensitive and specific for diagnosing CHPS. The ultrasound can detect a pyloric muscle thickness of ≥ 4 mm and a pyloric channel length of ≥ 16 mm, which are indicative of CHPS.

**Why Each Wrong Option is Incorrect**
**Option A:** Endoscopy is not the investigation of choice for CHPS as it may cause further irritation to the pyloric region and is not as accurate as ultrasound in measuring the pyloric muscle thickness.
**Option B:** Upper GI series is contraindicated in CHPS as it involves oral contrast and may cause vomiting, which can exacerbate the condition.
**Option C:** CT scan is not necessary for diagnosing CHPS and exposes the infant to radiation, which is undesirable in pediatric patients.

**Clinical Pearl / High-Yield Fact**
Remember that ultrasound is the preferred imaging modality for CHPS due to its non-invasive nature, high sensitivity, and specificity. This makes it an essential tool for pediatricians and radiologists to diagnose this condition accurately.

**Correct Answer:** C. CT scan