A 58-year-old man with chronic kidney disease presents with progressive proximal muscle weakness. Serum potassium is 7.8 mEq/L. Despite the membrane being depolarized, his muscles are less excitable. The best explanation is:

• A Sustained depolarization inactivates voltage-gated Na+ channels ✓
• B Hyperpolarization of the resting membrane potential away from threshold
• C Elevated extracellular K+ increases the K+ equilibrium potential
• D High extracellular K+ accelerates Na+-K+ ATPase beyond its capacity

Explanation

Severe hyperkalemia shifts the resting potential toward zero, but this sustained depolarization moves voltage-gated Na+ channels into the inactivated state, so they cannot reopen on demand. Excitability therefore falls despite depolarization, producing flaccid weakness. Option B describes hypokalemia, which paradoxically can also cause weakness by hyperpolarizing the membrane. Option C is wrong because raising extracellular K+ makes EK less negative, not more.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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