This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Introduction to Central Nervous System, Subject name: Physiology
Post-tetanic facilitation is thought to be the result of which of the following? 
Select and explain. 
A. Opening of voltage-gated sodium channels, B. Opening of transmitter-gated potassium channels, C. A buildup of calcium in the presynaptic terminal, D. Electrotonic conduction
C: A buildup of calcium in the presynaptic terminal
Answer: C: A buildup of calcium in the presynaptic terminal 
Explanation: Post-tetanic facilitation is the neuronal phenomenon in which a neuron is more easily excited after a brief period of activity. This phenomenon is thought to be due to the buildup of calcium in the presynaptic membrane caused by the prior neuronal activity. Subsequent neuronal impulses release neurotransmitter more readily as a result of this preplaced calcium from the prior stimulus.