A 52-year-old man with poorly controlled type 2 diabetes presents to the emergency department with 18 hours of rapidly worsening pain in his left calf after bumping his leg on a table. He reports fever, chills, and “the pain is unbearable.” Vital signs: T 39.2°C (102.6°F), BP 86/54 mm Hg, HR 128/min, RR 24/min. Physical examination shows a tense, swollen, erythematous calf that is warm to the touch. There is a small area of dusky discoloration with a few hemorrhagic bullae. The degree of tenderness is severe and appears out of proportion to the overlying skin findings. Distal pulses are intact. Labs show WBC 22,000/µL, ESR and CRP markedly elevated. CT of the leg demonstrates streaky gas tracking along the deep fascial planes.

Which of the following is the most likely diagnosis?