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

Prochlorperazine is an example of a phenothiazine used as an antiemetic because it functions primarily by blocking dopamine receptors in the brain, specifically in the area postrema, which is responsible for triggering the vomiting reflex. This receptor blockade effectively reduces nausea and vomiting, making it useful in various settings such as postoperative nausea, chemotherapy, and acute migraine treatment.

Phenothiazines, such as prochlorperazine, are known for their antipsychotic properties as well, but their ability to inhibit dopaminergic pathways in the central nervous system also lends them efficacy as antiemetic agents. In contrast, the other options listed serve different purposes. Dimenhydrinate is primarily an antihistamine used to prevent motion sickness, ondansetron is a selective serotonin receptor antagonist that is particularly effective for nausea related to chemotherapy, and metoclopramide is a prokinetic agent that also has some antiemetic properties but is not classified as a phenothiazine. Recognizing the specific classes of these medications helps in understanding their unique mechanisms and indications in antiemetic therapy.