Which vitamin deficiency is associated with increased homocysteine levels but normal methylmalonic acid levels?

The correct answer is associated with folate deficiency, which leads to increased levels of homocysteine. Folate plays a crucial role in the metabolism of amino acids, specifically in the remethylation of homocysteine to methionine. When there is a deficiency in folate, this process is impaired, resulting in elevated homocysteine levels.

In contrast, methylmalonic acid levels remain normal in folate deficiency because the metabolism of methylmalonic acid is primarily dependent on vitamin B12. A deficiency in vitamin B12 would typically cause both elevated homocysteine and elevated methylmalonic acid levels due to disruptions in different metabolic pathways. The fact that methylmalonic acid levels are normal indicates that vitamin B12 is likely adequate, thus supporting the diagnosis of folate deficiency.

Other vitamin deficiencies, such as those related to iron or thiamine, do not have a direct and established association with homocysteine metabolism. Iron deficiency chiefly affects hemoglobin synthesis and not amino acid metabolism, while thiamine deficiency is primarily involved in carbohydrate metabolism, not directly related to homocysteine or methylmalonic acid levels. Therefore, the clarity in the relationship between folate, homocysteine, and methylmal