A 68-year-old woman presents with sudden-onset syncope and palpitations while at rest. She has been on citalopram 40 mg daily for depression, ondansetron 8 mg three times daily for nausea, and recently completed azithromycin 500 mg daily for sinusitis. She also takes furosemide 40 mg daily. Laboratory studies reveal potassium 2.6 mEq/L, magnesium 1.2 mg/dL, and ionized calcium 8.1 mg/dL. ECG shows a markedly prolonged QTc of 620 msec with T-U wave fusion. On telemetry, she develops recurrent episodes of a polymorphic ventricular tachycardia. The structure marked **B** in the diagram—polymorphic QRS complexes twisting around the isoelectric baseline with continuously changing amplitude and polarity—is characteristic of which arrhythmia?