A 58-year-old man with megaloblastic anemia from vitamin B12 deficiency reports numbness of the feet and unsteadiness of gait. Neurological examination shows loss of vibration and joint position sense in the lower limbs along with extensor plantar responses. Which tracts are involved in this degenerative process?
- A Dorsal columns and lateral corticospinal tracts ✓
- B Spinothalamic tracts and spinocerebellar tracts
- C Corticobulbar tracts and medial longitudinal fasciculi
- D Anterior horn cells and dorsal roots
Correct answer: A. Dorsal columns and lateral corticospinal tracts
Explanation
Subacute combined degeneration of the cord in A12 deficiency affects the dorsal columns, causing loss of vibration and proprioception, and the lateral corticospinal tracts, causing spastic weakness with extensor plantars. Spinothalamic sensation (pain and temperature) is characteristically spared, which excludes option B. This tract selectivity distinguishes it from most other nutritional neuropathies and explains why folate deficiency, which corrects the hematological picture identically, does not produce these signs.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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