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: Medicine
[{'content': 'A toxin of clostridium tetanus, tetanospasmin causes muscular rigidity by blocking \nSelect and explain. \nA. Gamma amino butyric acid, B. Dopamine, C. Serotonin, D. Glutamine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]