A patient develops descending flaccid paralysis beginning with diplopia and dysphagia after consuming home-canned vegetables. Which step in neurotransmitter release is specifically blocked by Clostridium botulinum toxin?
- A Cleavage of SNARE proteins (synaptobrevin, SNAP-25, or syntaxin), preventing ACh vesicle fusion with the presynaptic membrane ✓
- B Entry of calcium through voltage-gated channels at the presynaptic terminal
- C Binding of ACh to nicotinic receptors on the postsynaptic membrane
- D Synthesis of acetylcholine from choline and acetyl-CoA
Correct answer: A. Cleavage of SNARE proteins (synaptobrevin, SNAP-25, or syntaxin), preventing ACh vesicle fusion with the presynaptic membrane
Explanation
Botulinum neurotoxin (BoNT) is a zinc-dependent endopeptidase that cleaves SNARE complex proteins: types A and E cleave SNAP-25, types B, D, F, G cleave synaptobrevin/VAMP, and type C cleaves both syntaxin and SNAP-25. This prevents ACh-containing synaptic vesicles from fusing with the presynaptic terminal membrane, causing flaccid paralysis at cholinergic (NMJ and autonomic) synapses. The toxin is a 150 kDa protein cleaved into heavy (binding) and light (catalytic) chains.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.