Sinoatrial Exit Block Mobitz Type II Sinus Node Dysfunction

A 68-year-old man with prior inferior myocardial infarction presents with recurrent dizziness and near-syncope. His ECG shows sinus P waves marching out regularly at a cycle length of 1080 ms. Telemetry reveals that a P wave abruptly fails to appear, producing a pause that measures exactly 2160 ms (precisely double the underlying P-P interval); the next P wave then arrives on time without progressive shortening of preceding P-P intervals. The rhythm pattern marked **B** in the diagram is most consistent with which underlying electrophysiologic mechanism?