A 28-year-old woman is admitted with palpitations and syncope. ECG shows a polymorphic ventricular tachycardia with QRS complexes that appear to twist around the isoelectric baseline, as marked **A** in the diagram. Her QTc is 520 ms. She denies recent drug use but takes citalopram 60 mg daily for depression. Which of the following best describes the mechanism underlying the arrhythmia pattern marked **A**?