A 15-year-old girl with anorexia nervosa has a BMI of 13 kg/m², secondary amenorrhea for 9 months, lanugo hair, low estrogen, and an ECG showing a QTc of 510 ms. Which metabolic/electrolyte abnormality is most immediately life-threatening in this patient?

• A Hyponatremia from water loading
• B Hypoglycemia from glycogen depletion
• C Hypokalemia causing QTc prolongation and arrhythmia ✓
• D Hypophosphatemia from refeeding syndrome

Explanation

In anorexia nervosa, hypokalemia (from purging behavior, laxative abuse, or severe malnutrition) is the most immediately life-threatening electrolyte abnormality because it causes QTc prolongation (as seen: QTc 510 ms) predisposing to ventricular arrhythmias including torsades de pointes and sudden cardiac death. Cardiac arrhythmia is the primary cause of mortality in anorexia nervosa. Hypophosphatemia from refeeding syndrome is a risk upon initiating nutrition, not the immediate threat at presentation. Hyponatremia and hypoglycemia may occur but are less common immediate threats than the cardiac risk from hypokalemia.

Reference: Ghai Essential Pediatrics, 10th ed.

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