This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
Which of the following is not a pyrimidine base? 
Select and explain. 
A. Cytosine, B. Uracil, C. Guanine, D. Thymine
C: Guanine
Answer: C: Guanine 
Explanation: Guanine is a purine base Ref-Harpers illustrated biochemistry 30/e p329