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MCAT Question A Day - 8/1/13 - Answer!

Depolarization takes place when _____ ions enter the cell from the outside, whereas repolarization begins when _____ ions leave the cell from the inside.

A.  K+, Na+
B.  K+, K+
C.  Na+, Na+
D.  Na+, K+

The correct answer is (D). The passage of Na+ into the cell from outside is what causes the reversal of the resting potential from (+) outside the membrane and (-) inside the membrane to (-) outside and (+) inside (depolarized state). The release of K+ from the inside at the onset of repolarization then helps return the cell membrane to its resting potential.