A 60-year-old man who had been treated for lung cancer com- plains of a rash on his chest and pain in his upper arms and calves. He cannot raise his arms and climbs the stairs only with difficulty. A muscle biopsy shows perivascular infiltrates of lymphocytes and plasma cells extending in between the muscle fibers. Immunofluorescence reveals immune complexes in the walls of intramuscular blood vessels. Which of the following is the most likely diagnosis?