Botulinum toxin causes flaccid paralysis by cleaving specific proteins at the presynaptic nerve terminal. Which step of neuromuscular transmission is blocked?

• A Calcium entry through voltage-gated channels
• B Degradation of acetylcholine by acetylcholinesterase
• C Binding of acetylcholine to postsynaptic nicotinic receptors
• D Fusion of acetylcholine vesicles with the presynaptic membrane ✓

Explanation

Botulinum toxin is internalised into the terminal and cleaves SNARE proteins such as SNAP-25, syntaxin, or synaptobrevin, which are required for docking and fusion of acetylcholine vesicles with the presynaptic membrane. Calcium entry still occurs normally, distinguishing it from Lambert-Eaton syndrome. Postsynaptic receptors and acetylcholinesterase are untouched, which distinguishes it from myasthenia gravis and anticholinesterase poisoning respectively.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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