Which lab finding is most specific for vitamin B12 deficiency?

• A. Low MMA
• B. Elevated MMA
• C. Normal MMA
• D. Elevated homocysteine

Answer: (B)

Vitamin B12 is needed for the enzyme that converts methylmalonyl-CoA to succinyl-CoA. When B12 is deficient, methylmalonyl-CoA builds up and is converted to methylmalonic acid, so MMA levels rise in blood and urine. This makes elevated MMA a specific indicator of B12 deficiency, because other common markers like homocysteine can rise in both B12 and folate deficiencies, reducing their specificity. Elevated homocysteine is more sensitive but less specific for B12 deficiency, since it also climbs with folate deficiency and other conditions. Normal MMA would argue against B12 deficiency, and while MMA can be affected by kidney function, among the given options it best points to B12 deficiency.