A 15-month-old child has received only breast milk since birth. He has pallor, a smooth beefy-red tongue, and absent deep tendon reflexes in both lower limbs. Peripheral smear shows macro-ovalocytes and hypersegmented neutrophils. Serum B12 is 65 pg/mL (normal >200). Which finding would MOST help differentiate neurological manifestations of B12 deficiency from folate deficiency?
- A Serum methylmalonic acid level ✓
- B Degree of macrocytosis on peripheral smear
- C Level of serum homocysteine
- D Subacute combined degeneration of the spinal cord with peripheral neuropathy
Correct answer: A. Serum methylmalonic acid level
Explanation
Both B12 and folate deficiency elevate homocysteine. However, methylmalonic acid (MMA) is specifically elevated only in B12 deficiency because B12 (adenosylcobalamin) is required for the methylmalonyl-CoA mutase reaction. Elevated MMA in the context of neuropathy confirms B12 deficiency. Subacute combined degeneration (SCD) also points to B12 deficiency but is a clinical finding, not a laboratory differentiator. Macrocytosis occurs in both. MMA is the most specific biochemical marker differentiating the two.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.