A 43-year-old cattle farm worker is brought to the surgeon with fever, malaise, and inflamed lesions on his hands and arms. He reports that about 2 weeks before his presentation al the hospital he noticed small painless, pruritic papules that quickly enlarged and developed a central vesicle. The vesicles developed into erosions and left painless necrotic ulcers with black, depressed eschar. Gram's staining of the ulcers reveals gram-positive spore-forming bacilli. Which of the following diseases is the most likely cause of these findings?