Drug Action Exam 1 Practice Test

An endogenous ligand is a molecule produced inside the body that binds to a receptor with some degree of specificity. This combination—originating within the organism and interacting with receptors in a selective way—defines physiologic signaling molecules like neurotransmitters and hormones. Saying it binds selectively emphasizes that these endogenous signals are tuned to particular receptor types, producing targeted responses rather than random, non-specific effects. Exogenous compounds (like drugs or toxins) come from outside the body, which is why the defining feature here focuses on internal production plus receptor-specific binding.