A 55-year-old man comes to the physician for increasing muscle weakness over the past 4 years. Serum creatine kinase is elevated. He takes no medications. There are no skin findings. Serum titers for antinuclear antibody and U1 RNP are negative. A muscle biopsy shows inflammatory cells with basophilic-rimmed vacuoles. Which of the following is the most likely explanation for these findings?
Inclusion body myositis
Fibromyalgia
Mixed connective tissue disease
Polymyalgia rheumatica
Polymyositis
