This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Basics of DNA, Subject name: Biochemistry
Which of the following base is usually not present in DNA? 
Select and explain. 
A. Adenine, B. Guanine, C. Cytosine, D. Uracil
D: Uracil
Answer: D: Uracil 
Explanation: Uracil is not usually present in DNA, instead thymine is present. However in DNA, spontaneous deamination of cytosine can produce uracil. But, this uracil is promptly recognized by Uracil DNA-glycosylase and replaced with cytosine. If uracil were present in DNA, DNA machinery could not differentiate the original uracil and the deaminated cytosine. Thus, presence of thymine/absence of uracil gives stability to DNA. Uracil is present in RNA in place of Thymine in DNA