Polymorphic VT — Long QT Torsades

A 62-year-old woman with depression (citalopram 40 mg daily) and recent respiratory infection (azithromycin) is admitted with acute gastroenteritis. She is given intravenous ondansetron for nausea. Twelve hours later, she collapses with sudden syncope. Telemetry captures the rhythm shown as **C** in the diagram — polymorphic ventricular tachycardia at 220 bpm with QRS complexes twisting around the baseline. Post-conversion ECG shows sinus rhythm with QTc 580 ms, prominent U waves, and T-wave alternans. Labs: K+ 2.9 mEq/L, Mg2+ 1.2 mg/dL. Which of the following is the FIRST-LINE pharmacologic therapy for the arrhythmia marked **C**?