What happens when a drug binds to a receptor?

When a drug binds to a receptor, it can effectively mimic or block the actions of endogenous regulatory molecules, which are naturally occurring substances in the body that bind to receptors and regulate various physiological processes. This interaction is crucial in pharmacology because it determines how a drug will exert its therapeutic effects or produce side effects.

When a drug mimics an endogenous molecule, it can activate the receptor and produce a similar effect—this is often seen with agonists, which enhance the receptor's activity. Conversely, if a drug blocks the receptor, it prevents the endogenous molecule from eliciting its effects—this is the role of antagonists, which inhibit the receptor's activity.

Thus, answer C captures the essence of how drug-receptor interactions function in pharmacodynamics, highlighting the ability of drugs to either stimulate or inhibit physiological responses depending on their nature and the receptors they target. This ability to influence biological functions is foundational to the therapeutic use of medications in clinical practice.