Answer & Explanation
The rhythm is sinus with an alternating QRS complex morphology – every other QRS is wide with a LBBB morphology. Further evaluation of the wide QRS complex also reveals a short PR interval – this is Wolff-Parkinson-White syndrome with a bypass tract that conducts intermittently. Thus it is a great comparison of a normal AV node conducted complex with the altered complex of Wolff-Parkinson-White (a “fusion” of AV node-His-Purkinje conduction with abnormal ventricular depolarization via a bypass tract).
The trick to this EKG is to recognize WPW as the cause for the wide complex beats – it might be tempting to diagnose bigeminy, or even, intermittent LBBB.