During the relative refractory period, an action potential can be elicited but only by a stimulus stronger than normal. This is because:

• A All sodium channels are closed and incapable of opening
• B The membrane potential is transiently more negative than the resting potential
• C Some sodium channels remain inactivated and potassium conductance is still elevated ✓
• D Calcium channels dominate the membrane conductance at this time

Explanation

Early in recovery some voltage-gated Na+ channels are still inactivated, reducing the available inward current, while voltage-gated K+ channels remain open, opposing depolarization. A supranormal stimulus therefore overcomes both factors. Option A describes the absolute refractory period, where no stimulus works. Option B describes the undershoot phase specifically and alone does not define the whole relative period, since residual inactivation is the decisive factor.

Reference: Ganong's Review of Medical Physiology, 27th ed.

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