Image of the Week: What comorbidity would cause this discolored bone?

A 61-year-old man presents with a 4-month history of progressive foot pain that causes difficulty walking. On examination, a firm and tender protuberance is noted at the first metatarsophalangeal (MTP) joint and dorsiflexion is limited. A few weeks later, as he is undergoing joint surgery, the surgeon notices discoloration of the bone. What comorbidity does this patient most likely have? ...