What characterizes a receptor antagonist?

A receptor antagonist is defined as a substance that binds to a receptor and blocks its activation. This means that when an antagonist occupies a receptor's binding site, it prevents the natural ligand or an agonist from activating that receptor. By doing so, receptor antagonists can inhibit the physiological effects that would normally occur when the receptor is activated.

Receptor antagonists are crucial in pharmacology, as they can be used therapeutically to modulate pathways where overactivation of a receptor could lead to adverse effects or disease symptoms. For instance, certain antagonists are employed to manage conditions such as hypertension, allergies, or even anxiety by dampening unwanted receptor activity.

The other options describe different types of interactions with receptors. A receptor agonist activates a receptor, boosting its activity, while a substance that competes for the same binding site but has no effect might be seen as a neutral antagonist rather than fulfilling the traditional definition of an antagonist, which emphasizes blocking activity. Therefore, the essence of a receptor antagonist lies in its blocking mechanism, which is precisely why option B accurately characterizes this class of substances.