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: Surgery
The mechanism of action of botulinum toxin A is best described by: 
Select and explain. 
A. Postsynaptic receptor blockade, B. Acetylcholinesterase inhibition, C. Presynaptic blockade of acetylcholine release, D. Slowing of myelinated nerve fiber transmission
C: Presynaptic blockade of acetylcholine release
Answer: C: Presynaptic blockade of acetylcholine release 
Explanation: Presynaptic blockade of acetylcholine release Botulinum toxin is an effective inhibitor of presynaptic acetylcholine exocytosis. Acting at the neuromuscular junction, cellular uptake is believed to occur through an active cell surface receptor-mediated process. This three-step chemical denervation is initiated by the toxin selectively binding to sites on the surface membrane of cholinergic nerve endings. An energy-dependent endocytotic process ensues internalizing the cell membrane with bound protein. The final step is presumed to act on calcium-mediated exocytosis, preventing the release of acetylcholine into the synaptic cleft. It is also believed that Botulinum toxin acts as a zinc-dependent metalloendo protease to inhibit exocytosis.