This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nerve, Subject name: Physiology
Refractory period of a myelinated nerve is due to: 
Select and explain. 
A. Voltage-dependence of Na+ channels, B. Time-dependence of Na+ channels, C. Voltage-dependence of K+ channels, D. Time-dependence of K+ channels
A: Voltage-dependence of Na+ channels
Answer: A: Voltage-dependence of Na+ channels 
Explanation: Only when the membrane has repolarized up to about - 40 mV to - 50 mV that the Na+ channels recover from their inactivated state and go into resting state. All this while, the membrane is refractory; that is, the membrane can excite again only when many of the Na+ channels have recovered (which itself depends on the membrane potential). Hence, refractory period is due to the voltage-dependance of the Na+ channels.