A 45-year-old woman complains of morning stiffness in the interphalangeal and metacarpophalangeal joints of both hands, followed by pain and swelling during movement after 4 months. Treatment with aspirin, other nonsteroidal anti-inflammatory drugs (NSAIDs), and gold compounds was ineffective, and her condition gradually worsened with fatigue, decreased appetite, weight loss, irregular fever, movement disorder in both elbow joints, elevated serum gamma globulin, rheumatoid factor (RF) positive (++), erythrocyte sedimentation rate (ESR) 60 mm at the end of the first hour, and elevated C-reactive protein (CRP). The most likely diagnosis is: