A 4-week-old first-born male presents with projectile non-bilious vomiting after feeds for 1 week. He remains hungry after vomiting but shows signs of dehydration and poor weight gain. On examination, an olive-sized mass is palpable in the epigastrium. Ultrasound confirms hypertrophic pyloric stenosis. Laboratory investigations reveal the electrolyte abnormality marked as **D** in the diagram. Which of the following is the MOST CRITICAL reason why this electrolyte derangement must be corrected BEFORE surgical intervention?

Explanation

## Why "Severe metabolic alkalosis suppresses the respiratory drive, increasing the risk of postoperative apnea" is right The anchor fact is that hypertrophic pyloric stenosis causes hypochloremic hypokalemic metabolic alkalosis due to loss of HCl-rich gastric contents. The clinical anchor explicitly states: "NEVER take to OR with severe alkalosis — risk of postoperative apnea (alkalosis suppresses respiratory drive)." This is the cardinal reason electrolyte correction must precede surgery. Alkalosis shifts the oxygen-hemoglobin dissociation curve leftward and depresses the respiratory center, making spontaneous breathing difficult postoperatively. Nelson 21e and Bailey & Love 28e both emphasize that preoperative fluid and electrolyte resuscitation is mandatory to avoid this life-threatening complication. ## Why each distractor is wrong - **Hypokalemia causes cardiac arrhythmias that are incompatible with general anesthesia**: While hypokalemia does increase arrhythmia risk, this is a secondary concern. The primary reason for preoperative correction is prevention of postoperative apnea from alkalosis, not arrhythmia prevention during anesthesia induction. - **Hypochloremia impairs gastric acid secretion and delays healing of the pyloromyotomy site**: This is physiologically incorrect. Hypochloremia does not impair healing of the pyloromyotomy; the surgical site heals well once the obstruction is relieved and normal feeding resumes. - **Alkalosis increases the risk of aspiration pneumonia during induction of anesthesia**: Alkalosis does not increase aspiration risk. The risk of aspiration is related to gastric contents and airway management, not acid-base status. **High-Yield:** In hypertrophic pyloric stenosis, always correct hypochloremic hypokalemic metabolic alkalosis BEFORE surgery to prevent postoperative apnea from respiratory depression. [cite: Nelson Textbook of Pediatrics 21e, Chapter 354; Bailey & Love's Short Practice of Surgery 28e]