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
Plateau phase of action potential of cardiac muscle starts with 
Select and explain. 
A. Inactivation of Na+ channel, B. Entry of Ca2+, C. Opening of K+ channel, D. Closure of K+ channel
B: Entry of Ca2+
Answer: B: Entry of Ca2+ 
Explanation: Plateau phase (Phase 2) starts with entry of Ca2+
Ca2+ entry is counterbalanced by efflux of K+. Hence, Plateau is maintained.