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
Mechanism of Acyclovir resistance - 
Select and explain. 
A. Thymidine kinase, B. DNA-dependent RNA polymerase, C. | spectrum, D. | Side effects
A: Thymidine kinase
Answer: A: Thymidine kinase 
Explanation: Ans. is 'a' i.e., Thymidine kinase Both HSV and VZV have been found to develop resistance to acyclovir during therapy the former primarily due to mutants deficient in thymidine kinase activity and the latter primarily by change in specificity of virus directed enzyme so that its affinity for acyclovir is decreased.