Which vitamin deficiency is responsible for subacute combined degeneration affecting dorsal columns and lateral corticospinal tracts?

• A. Vitamin B12 deficiency.
• B. Vitamin C deficiency.
• C. Vitamin D deficiency.
• D. Vitamin B1 deficiency.

Answer: (A)

Subacute combined degeneration is caused by vitamin B12 deficiency, which disrupts myelin maintenance in the spinal cord. Vitamin B12 acts as a cofactor for methionine synthase and methylmalonyl-CoA mutase, and when it is deficient, abnormal myelin accumulates and demyelination occurs. This preferentially affects the dorsal (posterior) columns, which carry vibration and proprioception, and the lateral corticospinal tracts, which mediate voluntary motor control. The resulting picture includes sensory ataxia with loss of vibration and position sense, a positive Romberg sign, and upper motor neuron–type signs like spasticity and hyperreflexia.

Clinically, people may also have megaloblastic anemia, and neurologic symptoms can improve with prompt vitamin B12 replacement, though some deficits may persist if treatment is delayed. The other vitamin deficiencies listed produce different syndromes: for example, vitamin C deficiency causes bleeding and connective tissue problems, vitamin D deficiency affects bone metabolism, and vitamin B1 deficiency leads to Wernicke-Korsakoff features or other neuropathies rather than the specific combined dorsal column and lateral CST demyelination.