A 64-year-old man from Mauritania was referred for evaluation of a multinodular tumor of the left foot. Foot lesions had been present for 38 years, starting on the left heel and gradually spreading to involve the whole foot and ankle, with walking impairment.
At the time that the patient was first seen, he was in a wheelchair. The patient sought medical care because of fatigue related to anemia of chronic disease (hemoglobin level, 9 g per deciliter). He did not report problems with his foot, being accustomed to this condition.
What could be the diagnosis ?