A 47-year-old homeless man presents to you for a diagnosis of a chronic illness. Five years ago he developed a painful sensory neuropathy involving both lower extremities. His B12 and folate levels were normal and he was prescribed pain medication. Four years ago he developed pruritic, violaceous plaques on his arms, legs, and chest treated variably and unsuccessfully with permethrin cream, ketoconazole, bacitracin ointment, oral levofloxacin and IV vancomycin. Three years ago he was admitted with a persistent rash and diarrhea treated with loperamide with improvement in the diarrhea. During the next two years he was repeatedly admitted for the rash, diarrhea, and confusion, treated variably with oral and intravenous antibiotics and anti-diarrheal agents with no improvement. The patient had a 33-year history of intravenous heroin use and hepatitis C infection. Pictures of his skin rash are shown below. What is your diagnosis and what test(s) would you order to confirm your suspicions?
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