This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic Bacteriology (Gram Positive Bacilli, Gram Negative Bacilli), Subject name: Microbiology
Mechanism of action of Tetanospasmin:- 
Select and explain. 
A. Inhibition of GABA release, B. Inhibition cAMP, C. Inactivation of Ach receptors, D. Inhibition of cGMP
A: Inhibition of GABA release
Answer: A: Inhibition of GABA release 
Explanation: Mechanism of action of Tetanospasmin is Inhibition of GABA release. C. tetani produces:- Tetanolysin:heat labile, oxygen labile hemolysin. It plays no role in the pathogenesis. Tetanospasmin or tetanus toxin (TT) : neurotoxin responsible for the pathogenesis of tetanus: - It is oxygen stable but heat labile; coded by plasmid. - Mechanism of action: Toxin acts pre-synaptically at the inhibitory neuron terminals and prevents release of inhibitory neurotransmitter GABA and glycine - leads to spastic muscle contraction. -Strychnine poisoning has a similar mechanism except that it acts post-synaptically. - BT (Botulinum Toxin) blocks the release of acetylcholine in neuromuscular junction, which leads to flaccid paralysis. Options 2, 3 4: Mechanism of Action of some impoant Bacterial Toxins: Heat Labile toxin of E.coli Increase cAMP Heat stable toxin of E.coli Increase cGMP Botulism toxin Inhibit release of acetylecholine from peripheral nerves Tetanus toxin Inhibit release of glycine and GABA at presynaptic terminals Diphtheria toxin Inhibit protein synthesis by inactivating EF 2