A female marathon runner without structural heart disease had a documented regular wide complex tachycardia (Fig. 25.1). At the time of electrophysiology (EP) testing, she had normal sinus and atrioventricular (AV) node function, no ventricular preexcitation, and no inducible ventricular tachycardia (VT). A wide complex tachycardia was induced with an atrial premature extrastimulus during sinus rhythm (Fig. 25.2). The QRS morphology was a right bundle branch block (RBBB) pattern with an inferior axis. There was not His-bundle potential recordable before the QRS during the tachycardia. What is the most likely diagnosis?
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