Enkephalin is an example of which type of molecule?

• A. Exogenous drug
• B. Endogenous hormone
• C. Endogenous opioid
• D. Enzyme

Answer: (C)

Enkephalin is a naturally occurring peptide that functions within the brain’s opioid system. It is produced inside the body, so it’s an endogenous substance, and it exerts its effects by binding to opioid receptors to modulate pain—producing opioid-like analgesia without introducing a drug from outside. That’s why the correct label is an endogenous opioid.

It isn’t an exogenous drug, since it isn’t introduced from outside the body. It isn’t an endogenous hormone, because while hormones travel through the bloodstream to distant targets, enkephalins act mainly as neurotransmitters/neuromodulators in the central nervous system. And it isn’t an enzyme, as it doesn’t catalyze a chemical reaction.