Okay, let's tackle this question. The scenario is a robust male baby with vigorous feeding and immediate vomiting at 2 months. The question is asking for the most probable diagnosis.

First, I need to recall the common causes of vomiting in infants. The key points here are the age (2 months), the presence of vigorous feeding, and immediate vomiting. Immediate vomiting after feeding could suggest a mechanical obstruction.

In infants, pyloric stenosis is a classic cause of projectile vomiting, usually around 2-4 months old. The baby is described as "robust," which might mean he's gaining weight despite the vomiting. Pyloric stenosis typically presents with non-bilious vomiting, and the pylorus becomes hypertrophic, leading to a palpable olive-shaped mass.

Another possibility could be gastroesophageal reflux, but that's less likely to present with immediate vomiting after feeding. Congenital pyloric atresia is a rare condition, but that usually presents with bilious vomiting and is more severe. Duodenal atresia would present with bilious vomiting as well, and the baby might be born with it, so by 2 months, maybe not the best fit.

So the most probable diagnosis here is hypertrophic pyloric stenosis. Let me check if the options include that. Since the user hasn't provided the options, but the correct answer is likely pyloric stenosis. The other options would be other causes like reflux, atresia, etc.

The core concept here is the pathophysiology of pyloric stenosis leading to obstruction. The immediate vomiting after feeding points to a mechanical blockage. The clinical pearl would be the classic presentation of pyloric stenosis in infants, especially the age and the nature of vomiting.

I need to structure the explanation with the required sections. Make sure to explain why the correct answer is right, why the others are wrong, and include the clinical pearl. Also, mention the key signs like the olive mass and non-bilious vomiting. The high-yield fact is the age range and the typical presentation.

**Core Concept**
The question tests recognition of **hypertrophic pyloric stenosis (HPS)**, a common cause of gastric outlet obstruction in infants. It arises from **hypertrophy of the pyloric sphincter**, leading to projectile, non-bilious vomiting after feeding. Classic presentation is in **2-3 month-old infants** with a **"robust" appearance** due to intermittent feeding.

**Why the Correct Answer is Right**
Hypertrophic pyloric stenosis presents with **non-bilious, projectile vomiting immediately after feeding** due to gastric outlet obstruction. The hypertrophied pylorus (often palpated as an "olive-shaped mass") prevents food passage. Despite vomiting, infants remain **alert and vigorous** ("robust"), with **weight gain** from high-protein intake. The 2-month age aligns with HPS onset, as peristalsis overcomes the hypertrophied muscle in younger infants. Diagnosis is confirmed by **ultrasound** (pylorus >14 mm in thickness).

**Why Each Wrong Option is Incorrect**
**Option A:** *Gastroesophageal reflux* causes **spillage or regurgitation**, not projectile vomiting, and is less likely to occur in a "robust"

✓ Correct Answer: D. Congenital hyperophic pyloric stenosis