A 68-year-old man presents with a 2-day history of worsening left leg pain, swelling, and redness below the knee. He felt generally unwell yesterday with mild fever. On examination, his left lower leg is erythematous, warm, tender, and swollen, with a reasonably well-demarcated border just below the tibial tuberosity. There are no obvious breaks in the skin, but he has some dry skin between his toes. What is the most likely diagnosis?