ECG CHALLENGEA 26-year-old male with no past medical issues presented with 24-hour sudden onset palpitations, chest discomfort, and dizziness.He was hemodynamically stable and his heart rate was 189 bpm, with no other remarkable findings on physical examination.A 12-lead ECG (Figure 1) showed regular wide complex tachycardia with right bundle-branch block and extreme superior axis.Stepwise management was provided with vagal maneuvers, escalating doses of adenosine, uptitrated intravenous verapamil, and cardioversion with serial discharges ≤270 J of biphasic energy.However, all interventions were ineffective, and the tachycardia remained incessant.What is the arrhythmia, and what cardiac structure is involved in its origin?Please turn the page to read the diagnosis.