Which statement distinguishes endogenous ligands from exogenous ligands?

• A. Endogenous ligands are larger molecules.
• B. Endogenous ligands are produced within the body, while exogenous ligands are introduced from outside (drugs)
• C. Exogenous ligands are produced within the body.
• D. Endogenous ligands are always inactive.

Answer: (B)

Endogenous ligands come from within the body, produced by our own cells and tissues (think neurotransmitters like dopamine or hormones like adrenaline). Exogenous ligands come from outside the body, such as drugs or environmental chemicals that can bind to receptors. The key distinction is source, not size or activity—endogenous ligands aren’t defined by being large, and they aren’t necessarily inactive. Exogenous ligands are simply external substances that can modulate receptor activity.