This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal tract, Subject name: Pharmacology
Ondansetron acts by: 
Select and explain. 
A. Action on CTZ, B. 5-HT3 antagonism, C. D1 and D2 receptor antagonism, D. Increasing GIT motility
A: Action on CTZ
Answer: A: Action on CTZ 
Explanation: Ondansetron blocks the depolarizing action of serotonin through 5-HT3 receptors on vagal afferents in the gut as well as in NTS and CTZ. It donot block dopamine receptors or ACH receptors.( ref KDT 6/e p646)