Certified Emergency Nurse (CEN) Practice Test 2025 - Free CEN Practice Questions and Study Guide

In the context of system response, an agonist is a substance that binds to a receptor and activates it, thereby stimulating a response. Agonists mimic the action of endogenous substances or neurotransmitters that normally interact with the receptor, leading to a biological response that supports or enhances a particular function or process in the body. This stimulation can increase cellular activity, promote physiological responses, or enhance the effects of a particular system.

The focus of the question is on how an agonist affects a system's response, which is why stimulation represents the primary action of an agonist. In contrasting terms, substances that diminish or inhibit system responses or reverse effects do not align with the role of an agonist, as these functions are characteristic of antagonists or inhibitors that block or reduce activity rather than promote it.

Thus, the correct understanding of an agonist is that it plays a critical role in activating specific pathways and eliciting an increase in function within the biological system.