This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
Cardiac muscles action potential at plateau is due to 
Select and explain. 
A. Influx of Na, B. Efflux of K, C. Influx of ca, D. Influx of cl
C: Influx of ca
Answer: C: Influx of ca 
Explanation: Ca2+ influx through more slowly opening Ca2+ channels (the Ca2+ current, ICA) produces the plateau phaseRepolarization is due to net K+ efflux through multiple types of K+ channelsThe initial depolarization is due to Na+ influx through rapidly opening Na+ channels Ref: Ganong's Review of Medical Physiology Twenty-Third Edition pg:491