Botulinum toxin prevents the release of what neurotransmitter at the neuromuscular junction for up to 5 months?

Botulinum toxin prevents the release of acetylcholine at the neuromuscular junction for up to 5 months.

Here is the step-by-step explanation:

1. Botulinum Toxin: This is a neurotoxic protein produced by the bacterium Clostridium botulinum.
2. Neuromuscular Junction: This is the synapse or junction between a motor neuron and a muscle fiber.
3. Neurotransmitter: A chemical substance that transmits signals across the synapse from one neuron to another or to a muscle fiber.
4. Acetylcholine: This is the specific neurotransmitter involved in muscle contraction.
5. Mechanism: Botulinum toxin inhibits the release of acetylcholine by cleaving proteins essential for the fusion of neurotransmitter-containing vesicles with the presynaptic membrane.
6. Duration: The effect of this inhibition can last for up to 5 months, leading to muscle paralysis.

Thus, the neurotransmitter whose release is prevented by botulinum toxin at the neuromuscular junction is acetylcholine.