Delta Wave WPW Pre-Excitation Short PR

A 22-year-old male presents with a three-year history of intermittent palpitations with abrupt onset and offset, lasting 15–60 minutes, often terminated by Valsalva maneuver. During a recent episode lasting 90 minutes, he developed near-syncope. His 12-lead ECG in sinus rhythm shows the classic triad of ventricular pre-excitation. The structure marked **A** in the diagram represents the slurred upstroke of the QRS complex. Which of the following best explains the mechanism underlying the formation of this delta wave in Wolff-Parkinson-White syndrome?