A 46-year-old man with longstanding alcohol dependence has eaten little during two weeks of vomiting. He is inattentive, walks with a broad-based gait and repeatedly reaches for objects beside the bed; examination shows horizontal nystagmus but no fever or focal weakness. His capillary glucose is 5.8 mmol/L (reference range 3.5–7.7 mmol/L), and there is no tremor or sweating. Which one of the following is the most likely diagnosis?