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

A. Abdominal ultrasound
B. Abdominal X-ray
C. Upper gastrointestinal contrast study
D. Blood tests

**Core Concept:**
Congenital hypertrophic pyloric stenosis (CHPS) is a condition that occurs in infants, characterized by the narrowing and thickening of the pyloric muscle layer leading to reduced gastric emptying and vomiting. Abdominal ultrasound (option A) is the most appropriate diagnostic tool for CHPS as it allows direct visualization of the pyloric muscle layer and helps in assessing the severity of the condition.

**Why the Correct Answer is Right:**
Ultrasound is a non-invasive, radiation-free method that can clearly visualize the pyloric muscle layer and assess the thickness, revealing the characteristic "double-outlet" sign in CHPS cases. This technique provides valuable information about the condition's severity and helps in guiding management decisions.

**Why Each Wrong Option is Incorrect:**
Abdominal X-ray (option B) is not as effective in diagnosing CHPS as ultrasound. It might only show dilated stomach but does not visualize the pyloric muscle layer directly. Blood tests (option D) are not specific to CHPS diagnosis and can be normal in cases of the condition.

**Clinical Pearl:**
Upper gastrointestinal contrast study (option C) is no longer the gold standard for CHPS diagnosis. Ultrasound has replaced it due to its non-invasiveness and ability to assess severity. However, contrast studies can still be useful in certain cases for further evaluation or excluding other differential diagnoses.

**Correct Answer:**
A. Abdominal ultrasound