A 17-year-old female with diabetes mellitus presents with pain in her left calf after a fall from her bicycle. She is febrile, tachycardic, looks ill, and her calf is tender to touch. The skin overlying the left leg has many bullae with black necrosis. There is a serosanguinous discharge. The distal pulses are decreased and there is crepitus. What is the next step in management?