A patient with chronic kidney disease has a serum potassium of 7.2 mEq/L. Compared with normal, his resting membrane potential in large motor neurons will be:

• A Hyperpolarized, because the potassium concentration gradient across the membrane has increased
• B Depolarized toward zero potential, with partial inactivation of voltage-gated sodium channels reducing excitability ✓
• C Unchanged, because sodium carries the dominant current at rest
• D Depolarized, with increased sodium channel availability producing hyperexcitability

Explanation

Resting potential depends mainly on the potassium gradient. Raising extracellular potassium shrinks that gradient, so the membrane depolarizes toward zero. Moderate depolarization brings many sodium channels into the inactivated state, so fewer are available for the upstroke and excitability falls, explaining the flaccid weakness of hyperkalemia. The distractor D tempts students who equate depolarization with excitation, ignoring steady-state inactivation of sodium channels.

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

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