Which type of molecule binds to a receptor site and stimulates a response in the cell?

The correct answer is indeed the agonist. An agonist is a type of molecule that binds to a receptor and activates it, leading to a physiological response in the cell. This interaction mimics the action of a naturally occurring substance, such as a neurotransmitter or hormone, enhancing the signaling process. By binding to the receptor site, the agonist effectively stimulates the cell’s activity and promotes a specific response, which can result in various effects depending on the type of receptor and the system in which it operates.

In contrast, antagonists are molecules that also bind to receptors but block or dampen the agonist-induced response. Inhibitors are more specific to enzyme activity, reducing the rate of reactions rather than stimulating receptor sites. Receptor blockers, a term often used interchangeably with antagonists, refer specifically to molecules that prevent the normal action of the receptor. Thus, agonists are the key players in promoting cellular responses, while the other choices serve roles in inhibiting or blocking those responses.