What type of drug binds to a receptor but does not activate it?

The correct answer is that a direct antagonist binds to a receptor but does not activate it. Direct antagonists, also known as blockers, attach to the same binding site as the natural ligand or neurotransmitter but prevent the receptor from being activated. This effectively inhibits the receptor's function, blocking the physiological effects that would normally be triggered by the agonist binding to the receptor.

In contrast, a direct agonist activates the receptor upon binding, producing a biological response. An indirect antagonist would interfere with the signaling pathways or systems that support receptor activation but does not directly bind to the receptor itself, while a noncompetitive agonist may bind to a site other than the active site and facilitate receptor activation in ways that do not involve direct competition with the natural ligand.