A 19-year-old woman has BMI 14.5 kg/m², intense fear of weight gain, amenorrhoea, and distorted body image. Which electrolyte abnormality is most dangerous and potentially life-threatening in this patient?

• A Mild hyponatraemia (Na 133 mEq/L)
• B Hypocalcaemia (Ca 8.0 mg/dL)
• C Hyperphosphataemia
• D Hypokalaemia (K 2.4 mEq/L) causing cardiac arrhythmia ✓

Explanation

Anorexia nervosa has the highest mortality rate of any psychiatric disorder (~5–10% per decade). The most dangerous cardiac complication is hypokalaemia-induced arrhythmia (including QTc prolongation, ventricular tachycardia/fibrillation). Hypokalaemia arises from both restrictive eating and purging behaviours (vomiting, laxative misuse). Additionally, refeeding syndrome (hypophosphataemia) is a life-threatening risk when nutritional rehabilitation is begun too rapidly. ECG monitoring and electrolyte correction are priority in medical management. BMI <13 or haemodynamic instability typically mandates medical/psychiatric inpatient admission.

Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.

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