A 52-year-old man with type 2 diabetes mellitus presents to the emergency department with 1 day of fever and a painful rash on his right lower leg. He reports rapid onset of localized pain and swelling after minor trauma while gardening. Temperature is 38.6°C (101.5°F). Examination shows a bright-red, warm, tender plaque on the anterior shin with raised, sharply demarcated borders. No fluctuance or purulent drainage is noted. Which of the following is the most likely causative organism?