Mr. W, 53 years old, is seen in the clinic with concerns about his left foot. He has a 40-year history of type 1 diabetes with fairly good control on twice-daily insulin. He denies injury but states that he tripped a few months previous and that his foot is sore when he walks. Physical examination reveals an edematous, erythemic, and warm foot. There is a superficial ulcer on the plantar surface. Which of the following is the most likely diagnosis? a. Fallen arch b. Arthritis c. Charcot joint d. Sprained ankle