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?

Dermatomyositis. Dermatomyositis is an immune-mediated microangiopathy that leads to obliteration of capillaries, ischemic injury, and muscle damage. Immunofluorescence demonstrates that the walls of many capillaries contain C5b-9 proteins (i.e., membrane attack complex). When dermatomyositis occurs in a middle-aged man, it is associated with an increased risk of epithelial cancer, most commonly carcinoma of the lung. By contrast, polymyositis and inclusion body myositis have only a chance association with malignancy. Lambert-Eaton myasthenic syndrome (choice C) is also seen in patients with lung cancer, but as with the other incorrect choices, it is not associated with a skin rash or muscle inflammation.Diagnosis: Dermatomyositis