A strict vegan develops progressive numbness and unsteadiness over months. Examination shows loss of vibration and joint position sense in the lower limbs, brisk knee jerks, extensor plantar responses, and pallor with MCV 112 fL. Serum vitamin B12 is 80 pg/mL. The lesion responsible for the neurological findings primarily affects which tracts?

• A Dorsal columns and lateral corticospinal tracts ✓
• B Spinocerebellar tracts and anterior horn cells
• C Spinothalamic tracts and dorsal columns
• D Lateral corticospinal tracts and spinothalamic tracts

Explanation

Subacute combined degeneration of the cord in B12 deficiency involves demyelination of the dorsal columns (loss of vibration, proprioception, sensory ataxia) and lateral corticospinal tracts (spastic weakness, hyperreflexia, Babinski sign). Spinothalamic sensation (pain and temperature) is spared, which eliminates options C and D. Accumulation of methylmalonic acid impairs myelin maintenance, and neurological damage may precede or occur without significant anemia, especially in dietary B12 deficiency.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.