A 28-year-old woman presents to the emergency department with recurrent syncope during emotional stress. Her 12-lead ECG shows the pattern marked **A** in the diagram — a polymorphic ventricular tachycardia with QRS complexes twisting around the isoelectric baseline, and a prolonged baseline QTc of 520 ms. She has no prior cardiac history, normal electrolytes, and is on no medications. Which of the following is the most likely underlying pathophysiological mechanism responsible for the arrhythmia pattern shown at **A**?