This is the answer and explanation for a USMLE practice question from the Dr. Ray Makar Facebook page. Try it first, then check your answer below.
The Question
A 4-week-old firstborn boy has forceful, nonbilious vomiting after every feed and seems hungry afterward. A firm epigastric mass is felt, and ultrasound shows a thickened pylorus. Cl– is 88, K+ 3.1 and HCO3– 34 mEq/L. What is the most appropriate next step?
A. Immediate pyloromyotomy
B. Upper GI contrast study
C. IV fluids and correction of electrolytes
D. Proton pump inhibitor and thickened feeds
E. Air enema
Correct Answer: C. IV fluids and correction of electrolytes
This is hypertrophic pyloric stenosis with the classic hypochloremic, hypokalemic metabolic alkalosis. Surgery (pyloromyotomy) is the definitive treatment, but it is never an emergency: the baby must first be rehydrated and have the electrolytes corrected. Operating while the infant is alkalotic risks postoperative apnea.
Why the Other Answers Are Wrong
- Immediate pyloromyotomy: Correct treatment, but only after fluids and electrolytes are corrected.
- Upper GI series: Not needed: ultrasound already confirmed the diagnosis.
- PPI and thickened feeds: Treatment for reflux, not for a mechanical obstruction.
- Air enema: Used for intussusception (currant jelly stools, sausage-shaped mass).
High-Yield Pearl
Firstborn boy, 3–6 weeks old, projectile nonbilious vomiting, olive mass = pyloric stenosis. Fix the alkalosis first, then operate.
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