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
All of the following drugs are protease inhibitors EXCEPT : 
Select and explain. 
A. Nelfinavir, B. Saquinavir, C. Abacavir, D. Ritonavir
C: Abacavir
Answer: C: Abacavir 
Explanation: None