53 李先生今年 65 歲，1 年前記憶力明顯減退，走路不穩，晚上常常跌倒。身體診查發現李先生兩腳無 力，下肢肌腱反射下降，出現 Babinski sign；雙腳的本體感覺及對音叉的震動感消失，但針刺感則正 常。李先生家中並沒有其他成員有和他一樣的症狀，也沒有接觸任何化學溶劑，未曾服用中藥， 5 年前曾接受全胃切除手術。下列那一項檢查對診斷最有幫助？
(A)腦電圖檢查（EEG）
(B)血中維他命 B12 濃度檢查
(C)腦部造影檢查（CT 或 MRI）
(D)腦脊髓液檢查

Subacute combined degeneration — Deficiency in vitamin B12 (cobalamin) leads to degeneration of the dorsal and lateral white matter of the spinal cord, producing a slowly progressive weakness, sensory ataxia, and paresthesias, and ultimately spasticity, paraplegia, and incontinence]. Not all patients with neurologic abnormalities will have anemia or macrocytosis. Supplemental treatment with vitamin B12 can stop progression and will produce neurologic improvement in most patients.

Nitrous oxide abuse can also lead to subacute combined degeneration, by inactivation of vitamin B12