A 59-year-old man presented with a 15-day history of fever, headache, leg myalgia, and weight loss. Physical examination showed no pathological findings. Initial diagnostic workup showed an increased concentration of C-reactive protein (16 mg/dL). An acute viral infection was suspected, and symptomatic treatment with oral analgesics and antipyretics was initiated, with clinical improvement noted 4 days after admission. However, fever and headache recurred 1 week after, and inflammatory marker concentrations were increased; a PET scan was done, which showed an increase in metabolic activity on the vascular walls of small and medium vessels. What is the likely diagnosis?
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