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
[{'content': 'Which of the following acts as an antagonist to NMDA receptor? \nSelect and explain. \nA. Ketamine, B. Spermine, C. Muscimol, D. Baclofen', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ketamine is a general anesthetic agent which acts by blocking excitatory NMDA type of glutamate receptors.', 'role': 'assistant'}]