A 6-week-old male infant presents with projectile, non-bilious vomiting after every feed for the past 2 weeks. He has a palpable olive-shaped mass in the right hypochondrium after vomiting. Lab shows metabolic alkalosis with hypokalemia and hypochloremia. What electrolyte and acid-base profile is MOST characteristic, and what is the mechanism?

• A Metabolic acidosis with high anion gap due to lactic acid from bowel obstruction
• B Hypochloremic, hypokalemic metabolic alkalosis due to persistent loss of HCl in gastric vomit with renal compensation retaining bicarbonate ✓
• C Respiratory alkalosis due to pain-related hyperventilation
• D Normal electrolytes with hyponatremia alone

Explanation

Hypertrophic pyloric stenosis causes forceful loss of hydrochloric acid (HCl) from the stomach. This results in hypochloremia and loss of H+ ions, creating metabolic alkalosis. As volume depletion worsens, the kidneys compensate by retaining sodium (exchanging it for H+, worsening alkalosis) and H+ rather than K+, creating hypokalemia. Paradoxical aciduria is seen late in severe dehydration. This electrolyte pattern — hypochloremic, hypokalemic metabolic alkalosis — is pathognomonic of pyloric stenosis. The definitive treatment is Ramstedt pyloromyotomy after electrolyte correction.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.