This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Central Nervous system, Subject name: Pharmacology
Which of the following acts as an antagonist to NMDA receptor? 
Select and explain. 
A. Ketamine, B. Spermine, C. Muscimol, D. Baclofen
A: Ketamine
Answer: A: Ketamine 
Explanation: Ketamine is a general anesthetic agent which acts by blocking excitatory NMDA type of glutamate receptors.