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A 30-year-old man with no structural heart disease has been evaluated for paroxysmal palpitation with documented regular narrow QRS tachycardia that has not responded to intravenous adenosine. Surface electrocardiogram has not shown any pre-excitation. He has been taken for an electrophysiology study after informed consent. Diagnostic catheters were placed at the coronary sinus, His bundle region, and right ventricle. During catheter manipulation a regular narrow QRS tachycardia with incomplete right bundle branch block morphology and normal QRS axis similar to the clinical tachycardia got induced. No other tachycardia was induced. What is the mechanism of tachycardia? Keywords: Incomplete right bundle branch block, Narrow QRS tachycardia, Upper septal ventricular tachycardia