A 52-year-old man from Delhi presents to the emergency department with sudden-onset weakness of both legs and inability to move his arms. His wife reports he was found unconscious 2 hours ago. On examination, he is alert but completely paralyzed except for eye movements. Blood pressure is 140/90 mmHg, heart rate 88/min, and respiratory rate 18/min. Serum potassium is 7.2 mEq/L (normal 3.5–5.0), and arterial blood gas shows pH 7.28. An ECG shows peaked T waves and prolonged PR interval. Which ion channel dysfunction best explains the loss of excitability in his skeletal muscles?