## **Core Concept**
The question presents a clinical scenario of a 26-day-old infant with symptoms of recurrent non-bilious vomiting, constipation, and weight loss. This combination of symptoms suggests a gastrointestinal issue that is likely related to a mechanical or functional obstruction.

## **Why the Correct Answer is Right**
The correct answer, **Pyloric Stenosis**, is a condition characterized by the thickening of the pylorus muscle, leading to gastric outlet obstruction. This condition typically presents in infants around 3 weeks of age with symptoms of non-bilious projectile vomiting, which can lead to weight loss and metabolic alkalosis. The vomiting is non-bilious because the obstruction is proximal to the ampulla of Vater, where bile enters the intestine. Constipation can also occur due to decreased fluid intake and loss of fluids through vomiting.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but based on the context, any condition not related to pyloric stenosis would be incorrect.
- **Option B:** If this option suggested another diagnosis such as gastroesophageal reflux disease (GERD), it would be incorrect because GERD typically presents with regurgitation and may have associated symptoms like irritability, but not usually with the severity of vomiting or the specific combination of symptoms seen in pyloric stenosis.
- **Option C:** If this suggested an intestinal obstruction (like intussusception or small bowel obstruction), it would be incorrect because these conditions often present with bilious vomiting due to the obstruction being distal to the ampulla of Vater.
- **Option D:** Without the specific option provided, any alternative diagnosis not aligning with pyloric stenosis would be considered incorrect based on the symptomatology provided.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl in this case is the classic presentation of pyloric stenosis: a "olive" mass palpable in the epigastric region, non-bilious projectile vomiting, and the typical age of presentation (2-8 weeks). The diagnosis can be confirmed with an ultrasound showing the thickened pylorus.

## **Correct Answer:** . **Pyloric Stenosis**