How does onabotulinumtoxinA (BOTOX) therapeutically produce paresis in overactive muscles?
A. Acts as an antagonist by binding to the ACh receptor on the postsynaptic membrane.
B. Rapidly degrades ACh in the synaptic cleft.
C. Facilitates the reuptake and sequestration of ACh into the presynaptic cell.
D. Disrupts the protein structure of the muscle cell receptor, thus preventing ACh from binding.
E. Inhibits the release of ACh from the presynaptic terminal at the neuromuscular junction.
ANS: E
Rationale: Botulinum toxin is naturally produced by a family of bacteria and, when ingested, causes widespread paralysis by inhibiting the release of ACh at the neuromuscular junction. When small doses of BOTOX are therapeutically injected directly into an overactive muscle, the inhibition of ACh release reduces or prevents contraction of the injected muscle.
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