A farm worker develops painful generalized muscle spasms and trismus after a contaminated puncture wound. The toxin responsible acts by which mechanism?
- A Cleaving synaptobrevin in inhibitory interneurons, preventing glycine and GABA release in the spinal cord ✓
- B Blocking calcium entry into motor nerve terminals, preventing acetylcholine release
- C Irreversibly binding nicotinic receptors at the neuromuscular junction
- D Inhibiting acetylcholinesterase at the neuromuscular junction
Correct answer: A. Cleaving synaptobrevin in inhibitory interneurons, preventing glycine and GABA release in the spinal cord
Explanation
Tetanospasmin travels retrogradely up motor axons to the spinal cord, enters inhibitory interneurons, and cleaves synaptobrevin (VAMP), a SNARE protein needed for vesicle fusion. Loss of glycinergic and GABAergic inhibition removes reciprocal and recurrent inhibition, producing unopposed alpha motor neuron discharge and spasms. Botulinum toxin uses a similar protease action but at the neuromuscular junction to block acetylcholine release, producing flaccid weakness, which distinguishes option B.
Reference: Ganong's Review of Medical Physiology, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.