Curare blocks action at acetylcholine synapses and causes paralysis. This drug is an example of an:

• A. enzyme inhibitor
• B. second messenger
• C. antagonist
• D. agonist

Answer: (C)

Verified step by step guidance

1. Understand the role of acetylcholine in the nervous system: Acetylcholine is a neurotransmitter that binds to receptors on the postsynaptic membrane, triggering a response such as muscle contraction.
2. Define the term 'antagonist': An antagonist is a molecule that binds to a receptor but does not activate it. Instead, it blocks the receptor, preventing the natural ligand (in this case, acetylcholine) from binding and triggering a response.
3. Analyze the effect of curare: Curare binds to acetylcholine receptors at synapses but does not activate them. This prevents acetylcholine from binding, which inhibits the transmission of nerve signals to muscles, leading to paralysis.
4. Differentiate between the options: An enzyme inhibitor blocks enzyme activity, which is unrelated to receptor binding. A second messenger is a signaling molecule inside the cell, which is also unrelated to curare's action. An agonist activates a receptor, which is the opposite of what curare does.
5. Conclude that curare is an antagonist because it blocks the action of acetylcholine at its receptor without activating the receptor itself.