A 65-year-old patient presents to you in general practice with a painful right lower leg. On questioning, he tells you that the pain has been present for a few months but he has not sought any medical advice for it. His past medical history is significant only for type 2 diabetes, for which he is on insulin. On examination he appears systemically well. Examination of the right lower limb shows a curvature of the tibia with overlying erythema, and palpation reveals it to be much warmer than the left leg.
Which ONE of the following conditions is the patient suffering from?