In suspected vitamin B12 deficiency, which additional lab finding supports the diagnosis besides low serum B12?

• A. Elevated methylmalonic acid
• B. Normal methylmalonic acid
• C. Elevated ferritin
• D. Low LDH

Answer: (A)

When you suspect vitamin B12 deficiency, a rise in methylmalonic acid is a key supportive finding because B12 is a cofactor for the conversion of methylmalonyl-CoA to succinyl-CoA. Without enough B12, methylmalonyl-CoA accumulates and is converted to methylmalonic acid, which levels rise in serum (or urine). This makes methylmalonic acid a more specific indicator for B12 deficiency and helps distinguish it from folate deficiency, which does not elevate methylmalonic acid (though both deficiencies can raise homocysteine). The other options aren’t as specific: ferritin can be influenced by inflammation and iron stores, and LDH is a non-specific marker that doesn’t pinpoint B12 deficiency.