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
The absolute refractory period of an action potential is due to: 
Select and explain. 
A. Insufficient neurotransmitter, B. Depletion of intracellular Na+, C. Inactivation of Na+ K+ ATPase, D. Voltage inactivation of the Na+ channel
D: Voltage inactivation of the Na+ channel
Answer: D: Voltage inactivation of the Na+ channel 
Explanation: None