A 38-year-old man with a 10-year history of alcohol use disorder presents to the emergency department with acute confusion, ataxia, and bilateral ophthalmoplegia (inability to abduct the eyes). His wife reports he has not eaten properly for 2 weeks. On examination, temperature is 37.2°C, heart rate 102/min, and he is disoriented to time and place. Which water-soluble vitamin deficiency is most likely responsible for this acute neuropsychiatric emergency?