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
Magnitude of action potential is determined by: 
Select and explain. 
A. Na+, B. Mg++, C. K++, D. Ca++
A: Na+
Answer: A: Na+ 
Explanation: Maximum magnitude is determined by equilibrium potential of Na+ ion.