Which of the following is an example of an exogenous ligand?

• A. A hormone produced by the body
• B. A neurotransmitter produced in the brain
• C. A drug that binds to a receptor
• D. A vitamin produced by the body

Answer: (C)

An exogenous ligand is a molecule that binds a receptor but comes from outside the body. A drug that binds to a receptor is a classic exogenous ligand because it is introduced from outside and interacts with the body's receptors to produce effects. In contrast, a hormone produced by the body or a neurotransmitter generated in the brain are endogenous ligands—they are synthesized inside the organism and regulate physiology by binding their natural receptors. A vitamin produced by the body would also be considered endogenous in this context. The main idea is origin: exogenous ligands originate externally, even though they can mimic or block the actions of the body's own ligands.