This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Initial rapid repolarization in cardiac muscle is mediated through 
Select and explain. 
A. Opening of Na channel, B. Opening of Ca channel, C. Opening of K+  channel, D. Closing of Na channel
C: Opening of K+  channel
Answer: C: Opening of K+  channel 
Explanation: 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.