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: Pharmacology
[{'content': 'Botulinum toxin blocks neuromuscular transmission by which of the following mechanism: \nSelect and explain. \nA. Closure of Ca++ channels at presynaptic membrane, B. Closure of Na+ channels at the postsynaptic membrane, C. Opening of K+channels at the presynaptic membrane, D. Opening of Cl– channels at the postsynaptic membrane', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Botulinum toxin acts by inhibiting the calcium mediated\xa0exocytosis of ACh from the vesicles in the synapse.', 'role': 'assistant'}]