Macrocytic anemia with neurologic symptoms; which metabolic marker is elevated in vitamin B12 deficiency but not in 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

Macrocytic anemia with neurologic symptoms; which metabolic marker is elevated in vitamin B12 deficiency but not in folate deficiency?

Explanation:
When a patient has macrocytic anemia with neurologic symptoms, you can differentiate vitamin B12 deficiency from folate deficiency by looking at specific metabolic byproducts. Methylmalonic acid depends on B12 for its conversion to succinyl-CoA. Without B12, methylmalonyl-CoA accumulates and is shunted to methylmalonic acid, so MMA levels rise. Folate deficiency doesn’t affect that conversion, so methylmalonic acid stays normal. Homocysteine, on the other hand, tends to rise in both B12 and folate deficiencies because the remethylation of homocysteine to methionine requires either B12 or 5-methyltetrahydrofolate; a deficiency in either can elevate homocysteine. Alanine and urate aren’t helpful markers for distinguishing these two deficiencies. So the metabolic marker elevated specifically in B12 deficiency but not in folate deficiency is methylmalonic acid.

When a patient has macrocytic anemia with neurologic symptoms, you can differentiate vitamin B12 deficiency from folate deficiency by looking at specific metabolic byproducts. Methylmalonic acid depends on B12 for its conversion to succinyl-CoA. Without B12, methylmalonyl-CoA accumulates and is shunted to methylmalonic acid, so MMA levels rise. Folate deficiency doesn’t affect that conversion, so methylmalonic acid stays normal.

Homocysteine, on the other hand, tends to rise in both B12 and folate deficiencies because the remethylation of homocysteine to methionine requires either B12 or 5-methyltetrahydrofolate; a deficiency in either can elevate homocysteine. Alanine and urate aren’t helpful markers for distinguishing these two deficiencies.

So the metabolic marker elevated specifically in B12 deficiency but not in folate deficiency is methylmalonic acid.

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