## **Core Concept**
Pyloric stenosis is a condition characterized by the narrowing of the pyloric canal, leading to gastric outlet obstruction. This condition commonly presents in infants but can also occur in adults. The investigation of choice aims to confirm the diagnosis and assess the severity of the obstruction.

## **Why the Correct Answer is Right**
The correct answer, **. Ultrasound**, is the investigation of choice for pyloric stenosis, especially in infants. Ultrasound is non-invasive, does not expose the patient to radiation, and can accurately measure the thickness of the pyloric muscle and the length of the pyloric canal. These measurements are crucial for diagnosing pyloric stenosis. In adults, while endoscopy and CT scans can also be used, ultrasound is often the first line due to its safety and efficacy.

## **Why Each Wrong Option is Incorrect**
- **Option A: . Endoscopy** - While endoscopy can visualize the pyloric region directly and is useful for assessing mucosal abnormalities, it may not provide information on the muscular thickness or the functional obstruction as effectively as ultrasound. It's more invasive compared to ultrasound.
- **Option B: . CT Scan** - CT scans can provide detailed images of the abdomen, including the pylorus, and can help in assessing the cause of gastric outlet obstruction. However, it involves radiation and is generally more expensive and less readily available than ultrasound.
- **Option D: . MRI** - MRI can provide detailed images of the gastrointestinal tract and surrounding structures without radiation. However, it's expensive, less readily available, and offers no significant advantage over ultrasound for the initial diagnosis of pyloric stenosis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in infants with pyloric stenosis, the typical ultrasound findings include a pyloric muscle thickness >3 mm and a pyloric canal length >15 mm. This information is critical for diagnosis and is a high-yield fact for exams.

## **Correct Answer: . Ultrasound**