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
Botulinum toxin blocks neuromuscular transmission by the following mechanism: 
Select and explain. 
A. 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
A: Closure of Ca++ channels at presynaptic membrane
Answer: A: Closure of Ca++ channels at presynaptic membrane 
Explanation: 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