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: Abacavir is nucleoside reverse transcriptase inhibitor and not a protease inhibitor. Nelfinovir, Titonavir and Saquinavir are all protease inhibitor. Ref: Antiretroviral Therapy By Erik De Clercq, 2001, Page 147 ; Harrison's Principles of Internal Medicine, Pages 1124, 1126.