Correct Answer: B

Rationale: Ondansetron is a serotonin 5-HT3 receptor antagonist commonly prescribed to prevent nausea and vomiting associated with chemotherapy, radiation therapy, or surgery. It works by blocking serotonin in the brain and gastrointestinal tract, which are key areas involved in triggering vomiting. In this scenario, the medication was given twice, likely following a scheduled regimen to maintain plasma levels and prevent emetic episodes throughout the day. The time gap between doses (8 hours) suggests proactive management of nausea rather than treatment of acute pain or infection. Other choices such as controlling blood pressure, reducing fever, or managing pain are unrelated to the pharmacologic action of Ondansetron, confirming that preventing nausea and vomiting is the most appropriate rationale for its use.

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