A 24-year-old woman comes to the emergency department because of palpitations that started suddenly 2 hours ago while she was exercising. She has had 4 similar episodes over the past year, each lasting 30 to 60 minutes before stopping abruptly. During previous episodes, she found that bearing down sometimes terminates the palpitations within seconds. She has no chest pain or shortness of breath and takes no medications. Her pulse is 180/min and regular. Physical examination shows no other abnormalities. ECG during the episode shows a regular narrow complex tachycardia. After the patient performs a Valsalva maneuver, the tachycardia terminates abruptly and sinus rhythm resumes immediately. A subsequent ECG in sinus rhythm shows a PR interval of 90 milliseconds and a slurred upstroke of the QRS complex. Which of the following mechanisms best explains this patient's arrhythmia?
Which of the following mechanisms best explains this patient's arrhythmia?