Which of the following nitrogen bases is not found in DNA?

Lysine is an amino acid, not a nitrogenous base, which is what the question is asking about. In the context of DNA, the nitrogen bases specifically refer to adenine, cytosine, guanine, and thymine. These bases are crucial components of the DNA structure, forming the rungs of the DNA ladder through complementary pairing (adenine with thymine and guanine with cytosine). Since lysine does not fit into the category of nitrogenous bases and is instead involved in protein synthesis, it is the correct answer to the question. Understanding the distinction between amino acids and nitrogenous bases is essential when studying molecular biology and genetics.