A 68-year-old man with a permanent dual-chamber pacemaker (implanted 3 weeks ago for complete heart block) presents to the emergency department with syncope. His 12-lead ECG shows regular pacing spikes at 70 bpm, but the structure marked **C** is evident: each spike is NOT followed by a corresponding QRS complex or P wave, and the underlying intrinsic rhythm (bradycardic at 35 bpm) is visible between spikes. Serum potassium is 7.8 mEq/L, and chest radiograph shows no lead dislodgement. Which of the following is the MOST likely cause of the malfunction shown at **C**?
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