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
[{'content': 'Which of the following is not a pyrimidine base? \nSelect and explain. \nA. Cytosine, B. Uracil, C. Guanine, D. Thymine', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Guanine is a purine base Ref-Harpers illustrated biochemistry 30/e p329', 'role': 'assistant'}]