A patient in their 70s with a history of paroxysmal atrial fibrillation and a structurally normal heart was prescribed flecainide, 100 mg twice a day, for rhythm control. The patient presented to the emergency department with complaints of dizziness. Their blood pressure was 60/44 mm Hg, heart rate was 166 beats/min, and oxygen saturation was 100% on ambient air. Serum sodium level was 138 mEq/L; serum potassium level, 4.4 mEq/L; and serum creatinine level, 0.86 mg/dL. The electrocardiogram (ECG) at presentation (Figure 1) showed a regular monomorphic wide complex tachycardia (WCT) at a rate of 160 beats/min.Question: What is the diagnosis?