Three days after admission for detoxification, a 32-year-old alcoholic man has ataxia, wide-based gait, bilateral abducens palsies, and horizontal nystagmus. Which vitamin deficiency is most likely?

• A. Niacin deficiency
• B. Thiamine deficiency
• C. Folate deficiency
• D. Vitamin B12 deficiency

Answer: (B)

Thiamine (vitamin B1) deficiency causing Wernicke's encephalopathy is the pattern here. In alcoholics, poor intake and impaired utilization lead to brain energy failure, especially in the mammillary bodies and brainstem. The hallmark presentation is a triad of gait ataxia with a wide-based stance, ocular motor abnormalities such as nystagmus and impaired horizontal eye movements (like bilateral abducens palsies), and confusion or mental status changes. This patient’s ataxia with a wide-based gait and horizontal nystagmus with CN VI dysfunction fits that ocular-motor component, making thiamine deficiency the most likely cause. Prompt thiamine administration is crucial to prevent progression to irreversible Korsakoff syndrome. Other vitamin deficiencies produce different clinical pictures (niacin with pellagra; folate or B12 with hematologic or neuropathic findings) and don’t typically present with this acute ocular-motor–gait syndrome in the setting of alcoholism.