A 45-year-old woman complains of morning stiffness in the interphalangeal and metacarpophalangeal joints of both hands. After 4 months, she develops pain and swelling during movement. Treatment with aspirin, other non-steroidal anti-inflammatory drugs (NSAIDs), and gold compounds has been ineffective, with progressive worsening of symptoms including 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: