Alimentary Tract Duplications

Explanation

While hypertrophic pyloric stenosis (A) classically presents with non-bilious projectile vomiting in a 3-6 week old infant, the key distinguishing feature here is the palpable abdominal mass. In pyloric stenosis, the 'olive' is typically felt in the epigastrium, not the RUQ. An alimentary tract duplication cyst often presents as a palpable mass and can cause obstructive symptoms like vomiting. Intussusception (C) classically presents with intermittent, colicky pain, currant-jelly stools, and a sausage-shaped mass, not projectile vomiting. Necrotizing enterocolitis (D) is a disease of prematurity and presents with a toxic, ill-appearing infant with bloody stools and pneumatosis intestinalis on imaging.