A strict vegan for eight years presents with progressive numbness of the feet, loss of ankle jerks, absent vibration sense in the toes, and bilateral extensor plantar responses. Hemoglobin is 11 g/dL with MCV 104 fL. Which pathological change underlies the neurological findings?

• A Demyelination confined to peripheral nerves without central involvement
• B Degeneration of dorsal columns and lateral corticospinal tracts of the spinal cord ✓
• C Symmetric necrosis of mammillary bodies and periaqueductal gray matter
• D Loss of Purkinje cells in the cerebellar vermis

Explanation

Vitamin B12 deficiency produces subacute combined degeneration: myelin vacuolation affecting the dorsal columns, causing loss of vibration and proprioception, and the lateral corticospinal tracts, causing weakness and extensor plantars. This central tract involvement is the key discriminator from folate deficiency, which causes identical megaloblastic anemia but NOT this spinal cord syndrome, and that fact kills option A, which describes a purely peripheral neuropathy. Option C describes Wernicke encephalopathy from thiamine deficiency.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.