<prompt> Botulinum toxin blocks neuromuscular transmission by which of the following mechanism:Closure of Ca channels at presynaptic membraneClosure of Na channels at the postsynaptic membraneOpening of Kchannels at the presynaptic membraneOpening of Cl channels at the postsynaptic membrane  <response>: Botulinum toxin acts by inhibiting the calcium mediated exocytosis of ACh from the vesicles in the synapse. 