CVS Practice Test 2025 – Complete Exam Preparation

During the non-pacemaker action potential, rapid repolarization primarily occurs in Phase 3. This phase is characterized by the opening of voltage-gated potassium channels, which allows potassium ions (K+) to flow out of the cell. As the K+ ions exit, the membrane potential becomes more negative, leading to repolarization following the depolarization that occurs in earlier phases of the action potential.

In contrast, the other phases involve different ionic movements that contribute to the action potential's characteristics. For instance, during Phase 0, sodium (Na+) channels open, causing rapid depolarization. In Phase 1, there is a brief repolarization due to the inactivation of sodium channels and transient outward current. Phase 2, known as the plateau phase, is marked by a balance of inward calcium (Ca2+) currents and outward potassium currents, maintaining a depolarized state rather than repolarization.

Therefore, Phase 3 is distinctly focused on the efflux of K+, leading to the rapid return of the membrane potential to its resting state, making it the correct answer.