A 9-month-old infant exclusively breastfed by a strictly vegan mother presents with megaloblastic anemia, irritability, hypotonia and a developmental regression. Serum folate is normal, serum B12 is extremely low. The mechanism of neurological damage in B12 deficiency is:

• A Direct neuronal apoptosis through oxidative stress
• B Cerebral ischemia from hyperhomocysteinemia-induced vasospasm
• C Interference with folate absorption in the gut
• D Impaired myelin synthesis due to accumulation of methylmalonyl-CoA and odd-chain fatty acids, and defective methionine synthesis ✓

Explanation

Vitamin B12 (cobalamin) is essential for two key enzymatic reactions: (1) conversion of methylmalonyl-CoA to succinyl-CoA (deficiency causes methylmalonic acidemia and accumulation of odd-chain fatty acids disrupting myelin); (2) conversion of homocysteine to methionine (deficiency impairs methionine and S-adenosylmethionine synthesis, critical for myelin maintenance and neurological function). Subacute combined degeneration of spinal cord is the hallmark in older patients. In infants, neurological regression, hypotonia and developmental delay are the clinical manifestations.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.