This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nerve Physiology, Subject name: Physiology
Magnitude of action potential in neuron, is determined by: 
Select and explain. 
A. Na+, B. Mg2+, C. K+, D. Ca2+
A: Na+
Answer: A: Na+ 
Explanation: Ans: A (Na+) Ref: Pal GK. Nerve. In: Textbook of Medical Physiology. Ahuja Publishing House. 2007; 18: 158.Explanation:Depolarization (upstroke) phase of action potential in a neuron is due to Na+ influxRepolarization is due to K+ effluxMg2+, Ca2+ are not directly involved in generation of action potential in nerve