A mutation in the MTHFR gene affects the metabolism of which vitamin?

The MTHFR gene is crucial for the metabolism of folate, which is a form of vitamin B9. Specifically, the MTHFR enzyme is responsible for converting 5,10-methylenetetrahydrofolate into 5-methyltetrahydrofolate, the active form of folate that participates in the remethylation of homocysteine to methionine. This process is vital for DNA synthesis, repair, and methylation, as well as overall cellular function.

Mutations in the MTHFR gene can lead to reduced enzyme activity, causing a buildup of homocysteine and decreased levels of the active form of folate in the body, which can have various health implications, including cardiovascular disease and issues during pregnancy.

The other vitamins mentioned—A, C, and D—are not directly affected by the MTHFR gene, as their metabolic pathways and functions are distinct from those related to folate. This makes folate the correct answer, as it is directly linked to the MTHFR gene's role in metabolism.