**Congenital Hypertrophic Pyloric Stenosis (CHPS) Diagnosis**

### 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. This condition is often diagnosed in the first few weeks of life and is a common cause of projectile vomiting in this age group.

### Why the Correct Answer is Right

The diagnosis of CHPS is primarily based on clinical presentation, which includes projectile vomiting, a palpable olive-shaped mass in the right upper quadrant, and gastric peristaltic waves. The correct answer is related to the diagnostic tool used to confirm the presence of this mass. The pyloric canal thickness can be measured using ultrasound, which is a non-invasive and accurate method for diagnosing CHPS. The normal pyloric canal thickness is less than 4mm, whereas in CHPS, it is greater than 4mm. This measurement helps to confirm the diagnosis and rule out other causes of vomiting.

### Why Each Wrong Option is Incorrect

**Option A:**
This option is incorrect as it does not relate to the diagnosis of CHPS.

**Option B:**
This option is incorrect as it is not a diagnostic tool for CHPS.

**Option C:**
This option is incorrect as it is a symptom of CHPS, not a diagnostic tool.

### Clinical Pearl / High-Yield Fact

A palpable olive-shaped mass in the right upper quadrant, known as the "pyloric olive," is a classic finding in CHPS. This mass is the thickened pyloric muscle and is often palpable during physical examination.

### Correct Answer:
**Correct Answer: A. Ultrasonography**