A 42-year-old man develops diplopia, dysarthria and progressively descending symmetric flaccid paralysis over 36 hours after consuming home-canned food. Sensory examination is normal and cerebrospinal fluid is unremarkable. The toxin responsible acts by:
- A Proteolytic cleavage of SNARE proteins required for acetylcholine release from the presynaptic terminal ✓
- B Irreversibly binding postsynaptic nicotinic receptors at the motor end plate
- C Blocking voltage-gated sodium channels along peripheral motor nerves
- D Accelerating acetylcholine hydrolysis at the neuromuscular junction
Correct answer: A. Proteolytic cleavage of SNARE proteins required for acetylcholine release from the presynaptic terminal
Explanation
Botulinum toxin is a zinc-dependent endopeptidase that cleaves SNARE proteins such as SNAP-25 and VAMP, preventing fusion of acetylcholine-containing vesicles with the presynaptic membrane. This produces a characteristic descending, afebrile, purely motor paralysis with normal sensation, beginning with cranial nerves. Curare-like agents act postsynaptically, tetrodotoxin blocks sodium channels, and accelerated acetylcholine breakdown is the mechanism of organophosphate excess, not deficit.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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