Absent right bundle branch block: Is it a clue of pre-excitation in Ebstein's anomaly?

A 14-year-old male with a history of symptomatic tachycardia was referred for ablation. Sinus rhythm electrocardiogram was not showing any pre-excitation. Tachycardia episode was showing antidromic tachycardia with left bundle branch block morphology. Echocardiographic examination was showing Ebstein's anomaly with septal tricuspid leaflet displaced 24 mm apically. The right bundle branch block (RBBB) was concealed during sinus rhythm. The RBBB was revealed with ablation of right posterior atriofascicular accessory pathway.