Final answer:
Methylmalonic acid (MMA) is elevated only in B12 deficiency and not in folate deficiency, as it is a compound that accumulates specifically when B12 is lacking because B12 is necessary for converting methyl malonyl-CoA to succinyl-CoA.
Step-by-step explanation:
The substance that is elevated only in B12 deficiency and not folate deficiency is methylmalonic acid (MMA). In contrast to homocysteine, which can be elevated in both B12 and folate deficiencies due to their roles in methylation reactions, MMA is a specific marker for B12 deficiency.
This is because MMA accumulates when B12 is deficient since vitamin B12 is required for the conversion of methyl malonyl-CoA to succinyl-CoA, a step in the metabolism of certain fatty acids and amino acids. Thus, elevated MMA levels in the blood can indicate a deficiency in B12 but not in folate.