Immediately after repolarization of a neuronal action potential, the membrane potential briefly becomes more negative than the normal resting potential before returning to baseline. This after-hyperpolarization is caused by:

• A Active pumping of potassium out of the cell by the Na-K ATPase
• B Persistent influx of sodium through inactivation-resistant channels
• C Continued efflux of potassium through slowly closing voltage-gated potassium channels ✓
• D Chloride entering the cell through ligand-gated channels opened by residual transmitter

Explanation

Voltage-gated potassium channels open more slowly than sodium channels and close sluggishly after repolarization. During this window, membrane permeability to potassium exceeds its resting value, driving the potential closer to the potassium equilibrium potential of about -90 mV, which is more negative than rest. The distractor A is tempting because the Na-K pump is electrogenic, but it contributes only a few millivolts and operates too slowly to shape this brief phase.

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

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