A patient presents to the emergency room with vomiting, diarrhea, high fever, and delirium. Upon physical examination, you notice large buboes, which are painful on palpation, and purpura and ecchymoses suggestive of disseminated intravascular coagulation. Gram stain on aspirate of a bubo reveals gram-negative rods with bipolar staining. Which of the following antibiotics is the drug of choice for empiric therapy?