**Question:** Congenital hypertrophic pyloric stenosis is diagnosed by

A. Abdominal ultrasound
B. Barium meal study
C. Blood tests
D. Esophagogastroduodenoscopy (EGD)

**Core Concept:**
Congenital hypertrophic pyloric stenosis (CHPS) is a condition in infants characterized by thickening and narrowing of the pyloric muscle, leading to gastric outlet obstruction. It presents with vomiting, poor weight gain, and feeding difficulties. The affected infants may also have abdominal distension and irritability.

**Why the Correct Answer is Right:**
A. Abdominal ultrasound (US) is the preferred diagnostic modality for CHPS as it can visualize the pylorus and assess its size and thickness. An ultrasound showing a pylorus larger than 10 mm in diameter is highly suggestive of CHPS.

**Why Each Wrong Option is Incorrect:**
B. Barium meal study (also known as barium swallow test) involves the ingestion of a contrast medium to visualize the gastrointestinal tract on x-rays. While it can detect pyloric stenosis, it is not as accurate as ultrasound in assessing the severity of pyloric stenosis.
C. Blood tests are generally ordered to evaluate anemia or other systemic conditions associated with CHPS. They do not directly diagnose the condition.
D. Esophagogastroduodenoscopy (EGD) is an invasive procedure where a flexible tube is inserted into the stomach for diagnostic and therapeutic purposes. While it can confirm the diagnosis of CHPS, ultrasound is a non-invasive, readily available, and safer method for diagnosis.

**Clinical Pearl:**
If CHPS is suspected clinically, an abdominal ultrasound should be performed as the first-line investigation. If the ultrasound findings align with the clinical suspicion, the procedure can be avoided, making it a safer and more cost-effective approach. However, if the ultrasound is normal, EGD should be considered to confirm the diagnosis.