This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Autonomic nervous system, Subject name: Pharmacology
[{'content': 'Botulinum toxin blocks neuromuscular transmission by 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: Ref:KD Tripathi pharmacology 7th edition (page.no: 99) Botulinum toxins A and B are highly potent exotoxins produced by Clostridium botulinum that is responsible for Botulism. These neurotoxic proteins cause long-lasting loss of cholinergic transmission by interacting with axonal protein involved in exocytic release of Ach and thereby closing the calcium channel in the pre synaptic membrane', 'role': 'assistant'}]