Which metabolic marker helps differentiate vitamin B12 deficiency from folate deficiency?

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Study for the NBME Form 13 Test with our comprehensive quiz collection, featuring flashcards and multiple-choice questions to ensure your readiness. Enhance your knowledge with detailed explanations for each question.

Multiple Choice

Which metabolic marker helps differentiate vitamin B12 deficiency from folate deficiency?

Explanation:
Methylmalonic acid helps distinguish vitamin B12 deficiency from folate deficiency. Vitamin B12 is a cofactor for the enzyme that converts methylmalonyl-CoA to succinyl-CoA; when B12 is deficient, methylmalonic acid accumulates and rises in the blood. Folate deficiency does not affect this pathway, so methylmalonic acid remains normal. In contrast, homocysteine levels can be elevated in both B12 and folate deficiencies because both nutrients are needed for homocysteine remethylation, so it’s less helpful for differentiation. Serum ferritin is unrelated to these pathways. Therefore, elevated methylmalonic acid points to B12 deficiency, whereas normal methylmalonic acid with elevated homocysteine would suggest folate deficiency.

Methylmalonic acid helps distinguish vitamin B12 deficiency from folate deficiency. Vitamin B12 is a cofactor for the enzyme that converts methylmalonyl-CoA to succinyl-CoA; when B12 is deficient, methylmalonic acid accumulates and rises in the blood. Folate deficiency does not affect this pathway, so methylmalonic acid remains normal. In contrast, homocysteine levels can be elevated in both B12 and folate deficiencies because both nutrients are needed for homocysteine remethylation, so it’s less helpful for differentiation. Serum ferritin is unrelated to these pathways. Therefore, elevated methylmalonic acid points to B12 deficiency, whereas normal methylmalonic acid with elevated homocysteine would suggest folate deficiency.

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