Which of the following is NOT a main type of antiemetic agent?

The choice of beta blockers as the option that is not a main type of antiemetic agent is based on the classification and primary action of various antiemetic medications.

5-HT3 receptor antagonists are specifically used to block serotonin receptors in the gastrointestinal tract and central nervous system, effectively preventing nausea and vomiting often associated with chemotherapy and post-operative settings. Dopamine antagonists also play a significant role in the management of nausea and vomiting by blocking dopamine receptors, which are involved in the vomiting reflex. Antihistamines, on the other hand, are utilized to combat nausea and vomiting particularly related to motion sickness and vestibular disturbances by blocking histamine receptors that can trigger these symptoms.

In contrast, beta blockers do not have a direct application in the prevention or treatment of nausea and vomiting. Their primary role is in cardiovascular conditions, where they are prescribed to manage issues like hypertension, angina, and arrhythmias. Thus, they do not fit into the category of agents primarily aimed at controlling nausea and vomiting.