ECG — Hyperkalemia (Peaked T Waves)

A 68-year-old man with stage 4 CKD (eGFR 22 mL/min) on lisinopril and spironolactone presents to the emergency department with palpitations and weakness. His serum potassium is 6.8 mEq/L. An ECG is performed and shows the characteristic findings marked in the diagram. The structure marked **A** — tall peaked (tented) T waves with narrow base, most prominent in the precordial leads — represents the earliest and most characteristic ECG manifestation of his condition. Which of the following best describes the underlying cardiac electrophysiological mechanism responsible for this ECG finding?