Which of the following is an example of a dopamine antagonist used as an antiemetic?

Metoclopramide is a well-known dopamine antagonist that is effective in treating nausea and vomiting. It works primarily as an antagonist at the D2 receptors in the brain, particularly in the area postrema, which is known as the trigger zone for emesis (vomiting). By blocking dopamine receptors, metoclopramide helps to control nausea and prevent vomiting, making it a useful agent in various situations, such as motion sickness, postoperative nausea, and nausea associated with chemotherapy.

In addition to its antiemetic properties, metoclopramide has gastroprokinetic effects, enhancing gastric emptying and intestinal motility. This multifaceted action contributes to its effectiveness in managing nausea and vomiting, particularly when these symptoms are related to gastrointestinal issues.

Other options listed, such as ondansetron, are serotonin antagonists and operate through different mechanisms. Dronabinol is a cannabinoid that works primarily through the endocannabinoid system, and diphenhydramine is an antihistamine that primarily acts as an H1 receptor blocker. Therefore, metoclopramide distinctly stands out as a dopamine antagonist in the context of antiemetic therapy.