Hypokalemia — U Waves and Long QT

A 32-year-old woman with a 5-year history of bulimia nervosa presents with generalized weakness, palpitations, and muscle cramps. Laboratory investigations reveal serum potassium 2.4 mEq/L and serum magnesium 1.3 mg/dL. Her 12-lead ECG shows the pattern marked **A** in the diagram: flattened T waves, prominent U waves (best seen in V2–V4), mild ST-segment depression, and apparent QT prolongation. She subsequently develops a brief run of polymorphic ventricular tachycardia. Which of the following is the MOST appropriate initial management step?