Narrow QRS Tachycardia: What Is the Mechanism?

A 45-year-old man was referred for radiofrequency catheter ablation of narrow QRS tachycardia that terminated with intravenous adenosine. A 12-lead electrocardiogram showed no baseline pre-excitation. The echocardiogram was essentially normal. The electrophysiological study showed a normal atrial-His interval of 110 ms and a His-ventricular interval of 44 ms during sinus rhythm. An anterograde study demonstrated no dual atrioventricular nodal physiology. Atrial pacing protocols easily and reproducibly induced a narrow QRS tachycardia. What is the mechanism of the tachycardia?