Which of the following describes an antagonist?

An antagonist is defined as a substance that binds to a receptor but does not activate it, preventing other substances (such as agonists) from eliciting a physiological response. This is crucial in pharmacology, as antagonists can block or inhibit the action of agonists (which are the substances that activate receptors to produce a response). By failing to stimulate a response while still occupying the receptor site, antagonists can effectively diminish or nullify the effects of naturally occurring substances or medications.

In contrast, other options do not accurately describe an antagonist. Options focusing on promoting physiological responses or enhancing absorption of nutrients pertain to mechanisms associated with agonists or other types of drugs, but do not capture the essence of an antagonist's function. Furthermore, the notion that an antagonist is always more effective than agonists is misleading, as efficacy can vary widely depending on the context of use and the specific drugs involved. Thus, the choice that describes an antagonist accurately is the one that notes its interaction without causing a physiological response.