A 40-year-old man develops blurred vision, ptosis, descending flaccid paralysis, and respiratory failure 18 hours after eating home-canned food. The toxin responsible acts by which mechanism?
- A Blocking acetylcholine receptors at the neuromuscular junction
- B Cleaving SNARE proteins and preventing fusion of acetylcholine vesicles with the presynaptic membrane ✓
- C Inhibiting choline acetyltransferase in the motor nerve terminal
- D Opening voltage-gated potassium channels and hyperpolarizing the nerve terminal
Correct answer: B. Cleaving SNARE proteins and preventing fusion of acetylcholine vesicles with the presynaptic membrane
Explanation
Botulinum toxin is a zinc endopeptidase taken up by the motor nerve terminal, where it cleaves SNARE proteins such as syntaxin, SNAP-25, or synaptobrevin. Vesicles cannot fuse with the presynaptic membrane, so acetylcholine release fails, giving flaccid paralysis with normal receptor function. Option A describes myasthenia gravis and curare-like drugs, where transmission improves with repetitive stimulation rather than failing completely.
Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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