Q. A 55-year-old man with a history of chronic alcohol use disorder is brought to the emergency department by his friends. They report that he has been increasingly confused over the past few days. On examination, the patient is disoriented to time and place. He also has difficulty walking due to poor coordination and displays nystagmus and a lateral gaze palsy. Neurological imaging is non-diagnostic. What is the nutritional deficiency likely responsible for this patient's condition?