A 45-year-old man with a 20-year history of alcohol use disorder presents to the emergency department with acute confusion, horizontal nystagmus, and inability to abduct both eyes. On examination, he is ataxic and disoriented to time and place. The structure marked **B** in the diagram is deficient, and its active form (thiamine pyrophosphate) is essential for the function of several metabolic enzymes. Which of the following is the MOST critical immediate management step in this patient?