What is an agonist?

An agonist is defined as a molecule that stimulates a response by binding to a receptor. In pharmacology, agonists are compounds that, upon binding to a receptor, activate it and produce a biological response. This activation can lead to a variety of physiological effects depending on the type of receptor and tissue involved.

For example, when an agonist binds to a receptor in the heart, it may increase heart rate, whereas binding to a receptor in the lungs may help in bronchodilation. The effectiveness of an agonist is determined by its affinity for the receptor and its intrinsic activity, meaning how well it activates the receptor once bound.

The other choices refer to different mechanisms of interactions with receptors, such as blockers or antagonists that inhibit normal receptor activity, or inactive substances that do not modify receptor functionality. Therefore, option B accurately characterizes the role of an agonist in pharmacological terms.