Wide complex tachycardia with negative precordial concordance: All that glitters is not gold

A 38?year?old woman with recurrent palpitations and no structuralheart disease presented with a documented wide QRS complextachycardia (WCT) with negative concordance (Figure 1). There wasno significant past medical history, and the clinical examination wasutterly unremarkable. A standard electrophysiologic study wasperformed using multipolar?electrode catheters positioned in thehigh right atrium, His bundle, right ventricular apex, and coronarysinus. Parihisian pacing was compatible with a nodal response. Then,spontaneously, the induced narrow?QRS tachycardia (NCT) (Figure 2)was followed by a WCT (Figure 3), identical to the patient's clinicaltachycardia (Figure 1). What is the tachycardia mechanism?