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
Botulinum toxin blocks neuromuscular transmission by which of 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: Botulinum toxin acts by inhibiting the calcium mediated exocytosis of ACh from the vesicles in the synapse.