Which vitamin deficiency is most likely to result in impaired DNA replication?

The correct answer is related to the significant role that folic acid plays in DNA replication and synthesis. Folic acid, also known as vitamin B9, is crucial for the synthesis of nucleotides, which are the building blocks of DNA. It serves as a cofactor for enzymes involved in the methylation of nucleotides, particularly during the formation of purines and pyrimidines. A deficiency in folic acid can lead to impaired DNA replication and repair mechanisms, resulting in issues such as megaloblastic anemia and potential complications during cell division.

In contrast, while vitamin B1 (thiamine) is essential for energy metabolism and the function of several enzymes, it does not have a direct role in DNA synthesis. Vitamin B3 (niacin) is important for energy production and the maintenance of healthy cells, yet its deficiency primarily affects metabolic processes rather than impacting DNA replication directly. Vitamin B12 is also involved in DNA synthesis, particularly in the conversion of homocysteine to methionine and in maintaining folate metabolism, but folate's direct influence on nucleotide synthesis makes it the most critical vitamin associated with DNA replication. Thus, a deficiency in folic acid specifically poses a more direct risk for impaired DNA replication.