Which of the following options is not a pyrimidine?

Adenine is classified as a purine, not a pyrimidine. The distinction between purines and pyrimidines is important in biochemistry, particularly in the context of nucleotides, which are the building blocks of DNA and RNA. Pyrimidines are characterized by a single six-membered ring structure and include cytosine, thymine, and uracil, all of which are vital components of nucleic acids.

Cytosine, thymine, and uracil share the common structural element of a single-ring structure, making them part of the pyrimidine family. In contrast, purines like adenine are characterized by a double-ring structure, which differentiates them from pyrimidines. Understanding this classification aids in grasping the basic building blocks of genetic material and their functions in cellular processes.