A 73-year-old man who is an inpatient in a geropsychiatric inpatient unit is ataxic, has anterograde and retrograde amnesia, has a sluggish gaze, and has nystagmus. What is the most appropriate treatment?

Choice A reason: Memantine is used to treat moderate to severe Alzheimer’s disease by modulating glutamate activity. It is not appropriate for acute neurologic syndromes like Wernicke’s encephalopathy, which this patient’s symptoms suggest. Choice B reason: Thiamine deficiency is the hallmark of Wernicke’s encephalopathy, which presents with ataxia, ophthalmoplegia, and memory impairment. Prompt administration of thiamine is critical to prevent progression to Korsakoff syndrome and irreversible brain damage. Choice C reason: Donepezil is a cholinesterase inhibitor used for Alzheimer’s disease. It does not address the acute vitamin deficiency or neurologic symptoms seen in Wernicke’s encephalopathy. Choice D reason: Rivastigmine is another cholinesterase inhibitor used in dementia treatment. Like donepezil, it is not appropriate for thiamine deficiency-related neurologic syndromes.