Release of synaptic transmitter by exocytosis would be blocked most effectively by preventing the:

Preventing the flow of Ca2+ into the cell would prevent the release of transmitter, because Ca2+ initiates the intracellular events leading to the docking of the vesicle to its binding site on the active zone. Although Ca2+ normally enters the cell through voltage-operated channels that are opened by the depolarization of the nerve terminal that occurs as the action potential propagates along the nerve axon, release of transmitter will not occur if Ca2+ does not enter the nerve terminal. The flow of Na+ into the nerve terminal would depolarize the membrane and open Ca2+ channels, leading to Ca2+ entry and exocytosis. However, Na+ entry does not directly stimulate exocytosis. K+ does not affect the nerve terminal membrane.