Professional and Linguistic Assessments Board (PLAB) Practice Exam

Where does ondansetron primarily exert its action?

1. Cerebellum

2. Medulla oblongata

3. Thalamus

4. Hypothalamus

The correct answer is: Medulla oblongata

Ondansetron primarily exerts its action at the medulla oblongata, specifically targeting the area postrema, which is part of the central nervous system's vomiting center. This region is strategically located outside of the blood-brain barrier, making it highly sensitive to circulating emetogenic substances and neurotransmitters. By blocking serotonin receptors (specifically 5-HT3 receptors) in the area postrema, ondansetron effectively prevents nausea and vomiting otherwise triggered by a variety of stimuli, such as chemotherapy, radiotherapy, and postoperative states. This mechanism of action is crucial in providing relief from these distressing symptoms, which can significantly impact a patient's quality of life. While other brain regions mentioned have various roles in sensory processing and regulation of bodily functions, they are not the primary sites where ondansetron acts to counteract nausea and vomiting.