A 28-year-old man with renal failure is found to have a serum potassium of 7.0 mEq/L. The ECG shows peaked T waves. At the neuronal level, the most immediate effect of this potassium elevation on nerve excitability is:

• A Hyperpolarization of the resting membrane potential due to an increased potassium concentration gradient
• B Prolongation of the absolute refractory period by slowing potassium channel closure
• C Increased amplitude of the action potential because the sodium gradient is enhanced
• D Depolarization of the resting membrane potential toward threshold, initially increasing excitability ✓

Explanation

Extracellular potassium is a key determinant of the resting membrane potential because it sets the potassium equilibrium potential via the Nernst equation. Raising extracellular potassium reduces the outward potassium gradient, so the resting potential becomes less negative and moves closer to threshold, producing initial hyperexcitability. Option A states the opposite of the actual gradient change. With severe or sustained elevation, inactivation of sodium channels eventually reduces excitability, but the immediate effect is depolarization.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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