A 58-year-old man from Delhi presents to the emergency department with acute-onset syncope during exertion. On examination, he is haemodynamically stable with a regular heart rate of 45 bpm. ECG shows a prolonged PR interval (240 ms) with progressive lengthening until a P wave is not followed by a QRS complex. After the dropped beat, the PR interval resets and the cycle repeats. Which anatomical structure of the cardiac conduction system is most likely affected?