Phase 3 of the action potential is what?

• A. Depolarization Phase
• B. Repolarization Phase
• C. Plateau Phase
• D. Resting Phase

Answer: (B)

Phase 3 is the repolarization of the cardiac action potential. After the plateau, the inward calcium current wanes and voltage-gated potassium channels dominate, causing potassium to leave the cell. This outward K+ current drives the membrane potential back toward the negative resting level, ending the plateau. This phase is distinct from the initial depolarization that upstrokes the cell (phase 0) and from the plateau (phase 2) where Ca2+ influx keeps the membrane depolarized, as well as from the resting phase (phase 4) when the cell sits at its baseline potential until the next impulse.