A 9-month-old exclusively breastfed infant of a strict vegan mother presents with developmental regression (lost social smile, decreased head control), hypotonia, megaloblastic anemia, and methylmalonic aciduria. Serum vitamin B12 is undetectable. What is the primary mechanism of neurological damage in this infant?
- A Thiamine deficiency causing Wernicke's encephalopathy
- B Impaired myelin synthesis from methylmalonyl-CoA accumulation and methylation defect ✓
- C Zinc deficiency impairing neural progenitor differentiation
- D Folate deficiency preventing neural tube formation
Explanation
Vitamin B12 (cobalamin) deficiency causes neurological damage through two mechanisms: (1) Methylcobalamin is a cofactor for methionine synthase, converting homocysteine to methionine; deficiency impairs methylation reactions essential for myelin basic protein production and SAM (S-adenosylmethionine) generation, disrupting epigenetic regulation in neurons. (2) Adenosylcobalamin is required for methylmalonyl-CoA mutase; deficiency causes methylmalonyl-CoA accumulation, which is toxic to myelin. Both mechanisms combine to produce subacute combined degeneration of the cord in adults and devastating neurological regression in infants. Exclusively breastfed infants of vegan/vegetarian mothers are at risk because breast milk B12 reflects maternal stores.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.