What is an example of a drug that acts as an agonist?

An agonist is a substance that binds to a specific receptor and activates it, producing a biological response. Epinephrine is a prime example of an agonist because it binds to adrenergic receptors in the body, which are primarily responsible for the "fight or flight" response. When epinephrine interacts with these receptors, it stimulates various physiological effects, such as increased heart rate, dilation of air passages, and increased blood flow to muscles, all of which are adaptations that prepare the body for heightened activity and stress.

Other drugs listed do not function as agonists in the same way. For instance, fentanyl is a potent opioid agonist at mu receptors but in this context, epinephrine is the more recognized example due to its broad systemic effects and relevance in emergency medicine. Naloxone, on the other hand, is an opioid antagonist that blocks the effects of opioids rather than activating any receptors. Atropine serves as an anticholinergic, blocking the action of acetylcholine rather than mimicking its effects. Therefore, in the context of this question, epinephrine stands out as the drug that acts as an agonist by activating its respective receptors to produce significant bodily responses.