A 4-week-old previously healthy male infant, firstborn to a family with a history of pyloric stenosis, presents with projectile non-bilious vomiting after feeds and visible gastric peristaltic waves. On examination after nasogastric decompression, a firm, olive-shaped mass is palpated in the right upper quadrant. The structure marked **A** in the diagram represents the pathological finding. Which of the following is the MOST LIKELY etiology for the development of this lesion in this clinical context?

Question

Accepted Answer

B. Positive family history, male sex, firstborn status, and early exposure to macrolide antibiotics (e.g., erythromycin) in the first 2 weeks of life. ## Why Option 1 is correct

The structure marked **A** — the hypertrophied and hyperplastic circular smooth muscle of the pylorus forming the characteristic "olive" — develops in infantile hypertrophic pyloric stenosis (IHPS), the most common cause of gastric outlet obstruction in infants with peak incidence at 3–6 weeks of age. The clinical presentation (projectile non-bilious vomiting, hungry vomiter, visible peristaltic waves, palpable olive after decompression) combined with the demographic risk factors (male, firstborn, positive family history) and the critical risk factor of early macrolide antibiotic exposure (especially erythromycin in the first 2 weeks of life) directly identifies the etiology. Nelson Pediatrics and Pediatric Surgery NaT both emphasize that while the exact mechanism of pyloric muscle hypertrophy remains incompletely understood, these epidemiological and iatrogenic risk factors are well-established and modifiable.

## Why each distractor is wrong

- **Option 2 (Maternal hypercalcemia)**: Maternal hypercalcemia is associated with infantile hypercalcemia and nephrocalcinosis, not pyloric stenosis. There is no pathophysiological link between maternal calcium status and pyloric muscle hypertrophy.
- **Option 3 (Antral dysfunction)**: IHPS is a pyloric outlet obstruction, not an antral motility disorder. The pathology is localized hypertrophy of the pyloric circular muscle, not primary antral dysfunction. The antrum (proximal stomach, marked **C**) is dilated as a secondary consequence of the obstruction.
- **Option 4 (Aganglionosis)**: Congenital absence of ganglion cells (aganglionosis) is the pathology of Hirschsprung disease, which affects the distal colon and presents with enterocolitis or constipation, not gastric outlet obstruction in a 4-week-old with projectile vomiting.

**High-Yield:** IHPS peak incidence 3–6 weeks, male predominance 4–6:1, palpable "olive" after decompression, non-bilious projectile vomiting, and erythromycin exposure in first 2 weeks is a critical iatrogenic risk factor — always counsel against macrolides in neonates.

[cite: Nelson Pediatrics 22e; Pediatric Surgery NaT]

Answer

A. Congenital absence of ganglion cells in the myenteric plexus of the pylorus

Answer

C. Maternal hypercalcemia leading to increased gastric smooth muscle contractility

Answer

D. Delayed gastric emptying secondary to primary antral dysfunction

Explanation

Why Option 1 is correct

Why each distractor is wrong

Practice similar questions