A 19-year-old boy with rheumatic heart disease, post mitral valve replacement three years earlier, presented with recurrent episodes of palpitations. Sinus rhythm ECG showed left bundle branch block (LBBB). During symptoms, ECG showed regular wide complex tachycardia (WCT) at a rate of 140 beats per min (bpm) with QRS identical to sinus rhythm. Echocardiography revealed a dilated left ventricle with severely depressed systolic function. An electrophysiology study was performed. Four catheters - high right atrium, coronary sinus, right ventricle (RV), and multipolar catheter with close electrode spacing in the His region - were placed. The baseline His-Ventricle (HV) interval was 70 msec. Ventricular pacing showed central and decremental retrograde conduction. Clinical tachycardia at a rate of 140 bpm was initiated during atrial extra stimulus protocol with isoprenaline. The initiation and continuation of the tachycardia are shown (Figures1 and2). What is the best explanation for the arrhythmia seen? This article is protected by copyright. All rights reserved.