Repolarization is the return of the cell to resting state, caused by reentry of which ion into the cell while which ion exits?

• A. Sodium reentry into the cell
• B. Calcium influx into the cell
• C. Chloride efflux from the cell
• D. Potassium reentry into the cell while sodium exits the cell

Answer: (D)

Repolarization is driven by outward potassium current. After depolarization, voltage-gated potassium channels open and potassium ions move out of the cell, making the inside more negative and returning the membrane toward the resting potential. Sodium entry occurs during depolarization (and its channels inactivate later), while calcium movements help sustain the plateau but do not drive repolarization. So the key idea is potassium leaving the cell, not potassium reentry or sodium exiting during this phase.