Which electrolyte disturbance causes a prolonged QT interval that can lead to torsades de pointes?

• A. Hypermagnesemia
• B. Hypomagnesemia
• C. Hyperkalemia
• D. Hyponatremia

Answer: (B)

Prolonged QT interval signals delayed ventricular repolarization, creating a vulnerable window for dangerous rhythms like torsades de pointes. The electrolyte disturbance most closely linked to this is low magnesium. Magnesium helps stabilize cardiac cell membranes and modulates ion channels involved in repolarization, shaping the QT interval. When magnesium is deficient, early afterdepolarizations can occur during the prolonged repolarization, triggering torsades de pointes. Treatment with intravenous magnesium sulfate helps stabilize membranes and suppress these afterdepolarizations, even if magnesium levels are not severely low. Other electrolyte abnormalities have different cardiac effects and do not classically present with QT prolongation and torsades.