Which of the following substances blocks voltage-gated sodium channels?

• A. Atropine
• B. Tetrodotoxin
• C. Digoxin
• D. Ouabain

Answer: (B)

Verified step by step guidance

1. Understand the role of voltage-gated sodium channels: These channels are crucial for the initiation and propagation of action potentials in neurons. They open in response to changes in membrane potential, allowing sodium ions to enter the cell, which depolarizes the membrane.
2. Identify the substances listed: Atropine, Tetrodotoxin, Digoxin, and Ouabain. Each of these substances has different mechanisms of action and effects on the body.
3. Research the mechanism of action for each substance: Atropine is a muscarinic acetylcholine receptor antagonist, Digoxin inhibits the sodium-potassium ATPase pump, and Ouabain also inhibits the sodium-potassium ATPase pump.
4. Focus on Tetrodotoxin: This substance is known to specifically block voltage-gated sodium channels, preventing the influx of sodium ions and thereby inhibiting action potential propagation.
5. Conclude that Tetrodotoxin is the correct answer as it directly blocks voltage-gated sodium channels, unlike the other substances listed.