This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Which of the following 5HT3 receptor blockers has highest receptor affinity 
Select and explain. 
A. Ondansetron, B. Granisetron, C. Dolasetron, D. Palonosetron
D: Palonosetron
Answer: D: Palonosetron 
Explanation: Palonosetron 5 HT3 subtypes of receptors are impoant for - Emesis - Gut peristalsis - Bradycardia - Transient hypotension - Apnea - Itch 5 HT3 receptor antagonists are: - Ondensetron - Granisetron - Pahmosetron "Among the 5 HT3 receptor blockers palonosetron is a new i.v. agent that has greatest affinity for 5 HT3 receptor and has a long serum half life".