A 4-week-old male infant presents with projectile non-bilious vomiting after every feed. A firm, olive-sized mass is palpable in the right upper quadrant. Ultrasound shows pyloric muscle thickness of 4.5 mm and pyloric channel length of 18 mm. The metabolic derangement expected is:
- A Metabolic acidosis with hyponatremia
- B Metabolic alkalosis with hypochloremia and hypokalemia (paradoxical aciduria) ✓
- C Respiratory alkalosis with compensatory metabolic acidosis
- D Metabolic acidosis with hyperkalemia
Explanation
Hypertrophic pyloric stenosis causes repeated vomiting of gastric contents (HCl, KCl) without bile, resulting in: hypochloremic metabolic alkalosis with hypokalemia. The kidney attempts to conserve H+ (to correct alkalosis) by excreting K+ and paradoxically excreting acidic urine (paradoxical aciduria) despite systemic alkalosis. The diagnosis criteria on ultrasound: pyloric muscle thickness ≥4 mm and channel length ≥14 mm. Ramstedt's pyloromyotomy is the definitive treatment but is NOT an emergency — metabolic correction with 0.9% NaCl + KCl supplementation until chloride >100 mEq/L and CO2 <28 mEq/L is essential before surgery.
Reference: Bailey & Love's Short Practice of Surgery, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.