AV Nodal Reentrant Tachycardia (AVNRT)

A 28-year-old woman presents to the emergency department with sudden-onset palpitations, lightheadedness, and a sensation of neck pounding. Her vital signs show a regular heart rate of 160 bpm. 12-lead ECG during the episode reveals a narrow-complex regular tachycardia with retrograde P waves appearing immediately after the QRS complex, creating a pseudo-S wave in the inferior leads. The resting ECG between episodes is completely normal with no delta waves. The rhythm shown in the diagram marked **A** is suspected. Which of the following is the PRIMARY pathophysiological mechanism underlying this arrhythmia?