Initial rapid repolarization in cardiac muscle is mediated through: (A) Opening of Na channel, (B) Opening of Ca channel, (C) Opening of K+  channel, (D) Closing of Na channel
Answer is C. The main factor is opening of K+ channel. Although Ganong has maintained both closure of Na+ channels and opening of one type of K+ channel and closure of Na channel only the recent concept is K+ channel opening is the main reason of phase 1.