Which deficiency is specifically associated with subacute combined degeneration of the spinal cord?

• A. B12
• B. B1
• C. B6
• D. B9

Answer: (A)

Subacute combined degeneration arises when vitamin B12 is deficient, leading to demyelination of the dorsal columns and lateral corticospinal tracts in the spinal cord. Vitamin B12 is a cofactor for methionine synthase and methylmalonyl-CoA mutase; without it, myelin formation and maintenance falter, particularly in these spinal tracts. Clinically this produces a mix of sensory and motor signs: loss of vibration and proprioception with ataxia from posterior column involvement, plus spastic weakness and hyperreflexia from lateral tract damage. Patients may also have paresthesias, a positive Romberg, and sometimes macrocytic anemia or glossitis. This pattern is specifically linked to B12 deficiency, whereas deficiencies of other vitamins (like thiamine, B6, or folate) cause different neuropathies or neurological syndromes and do not produce the characteristic combined dorsal and lateral spinal cord demyelination seen here. Prompt B12 replacement is crucial to prevent progression and potential irreversible damage.