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: Biochemistry
[{'content': 'The term modified base refers to \nSelect and explain. \nA. Purine or pyrimidine attached to deoxyribose, B. A purine or pyrimidine that has been altered, C. A nitrogen containing ring other than a purine or pyrimidine that is part of a nucleotide, D. A purine or pyrimidine attached to a sugar by O-glycoside linkage', 'role': 'user'}, {'content': 'Answer: B \nExplanation: A modified base is one that has been modified after nucleic acid chain has been formed. These bases are modified by methylation, reduction, acetylation or glycosylation.\nIn t-RNA there are many modified bases, e.g. dihydrouridine, ribothymidine, pseudouridine, and 7-methylguanosine.\nIn DNA, the most common modified base is 5-methylcytosine.\nOther important modified bases are acetylcytosine and dimethyladenine.', 'role': 'assistant'}]