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
The term modified base refers to 
Select and explain. 
A. 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
B: A purine or pyrimidine that has been altered
Answer: B: A purine or pyrimidine that has been altered 
Explanation: 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.
In t-RNA there are many modified bases, e.g. dihydrouridine, ribothymidine, pseudouridine, and 7-methylguanosine.
In DNA, the most common modified base is 5-methylcytosine.
Other important modified bases are acetylcytosine and dimethyladenine.