Which mechanism describes the action of ondansetron in preventing nausea?

• A. Blocks serotonin (5-HT3) receptors
• B. Blocks dopamine D2 receptors
• C. Inhibits prostaglandin synthesis
• D. Antagonizes muscarinic receptors

Answer: (A)

Ondansetron works by blocking serotonin (5-HT3) receptors. These receptors sit on vagal afferents in the gut and in the chemoreceptor trigger zone of the brain. When serotonin binds them, it triggers signals that lead to nausea and vomiting. By antagonizing these receptors, ondansetron interrupts those signals at both peripheral and central sites, preventing nausea. This is different from blocking dopamine D2 receptors (a mechanism used by some other antiemetics), inhibiting prostaglandin synthesis (which reduces inflammation but isn’t the direct antiemetic signal), or antagonizing muscarinic receptors (which affects other nausea pathways).