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Disease Features Inflammatory diseases Rheumatoid arthritis Symmetrical joint involvement, autoantibody-positive, may see erosions on imaging in advanced disease Spondyloarthritis Low back involvement, sacroiliac joint tenderness, sacroiliitis on imaging RS3PE (remitting seronegative symmetrical synovitis with pitting edema) Peripheral edema, extensor synovitis on imaging, may be paraneoplastic Crystalline arthropathy Usually involvement of medium to large joints, intermittent symptoms, characteristic radiography and ultrasonographic findings, synovial fluid analysis positive for crystals Autoimmune myositis Muscle weakness and tenderness, elevated muscle enzymes Other connective tissue diseases Multiorgan involvement, specific autoantibodies may be positive, hypocomplementemia Noninflammatory diseases Osteoarthritis Pain exacerbated with use, normal inflammatory markers, degenerative changes on imaging Fibromyalgia Fatigue, chronic pain with more generalized involvement Spinal spondylosis and stenosis Numbness, paresthesias, muscle weakness, normal inflammatory markers Parkinson disease Muscle stiffness primary complaint, other symptoms typical of Parkinson disease including tremor and rigidity Infection Fever, heart murmur, leukocytosis, positive blood cultures Malignancy and paraneoplastic syndromes Weight loss, diffuse symptoms usually not limited to shoulder or pelvic girdle, lack of response to low-dose glucocorticoid therapy Drug-induced myopathy (eg, statin, glucocorticoid, colchicine) Lack of systemic symptoms, muscle weakness and tenderness, improvement with discontinuation of drug, elevated muscle enzymes, positive anti-HMG-CoA reductase antibody Thyroid and parathyroid disease Systemic symptoms typical of endocrinopathy; abnormal thyroid markers; abnormal calcium, phosphorus, or parathyroid levels