What is the action of a drug classified as an agonist?

• A. Prevents chemicals from reaching receptor sites
• B. Binds to a receptor and causes a specific response
• C. Binds to a receptor and prevents a response
• D. Opposes the receptor function

Answer: (B)

An agonist works by activating its receptor to produce a response. By binding to the receptor, it mimics a natural signaling molecule and triggers the cellular effect, which is exactly what is described here. For example, an agonist can turn on a receptor to initiate a downstream response, such as triggering a nerve signal or relaxing a muscle, depending on the receptor and tissue involved.

The other descriptions point to blocking or opposing receptor activity rather than activating it. Preventing chemicals from reaching the receptor sites stops activation rather than creating a response. Binding to a receptor and preventing a response describes an antagonist, not an agonist. Opposing receptor function also implies blocking or dampening the receptor’s action.