A 35-year-old woman consults a physician because of increasing discomfo in her hands over the past year. Her symptoms have developed insidiously and are often worst for the first hour after rising. She has also been experiencing vague symptoms of fatigue and malaise, paicularly in the early afternoon. Physical examination of her hands is notable for tenderness and synol thickening of most of the small hand joints of both hands, paicularly the proximal interphalangeal and metacarpophalangeal joints. A number of joints elsewhere, including in the feet, elbows, and ankles, are also symmetrically involved. An autoantibody directed against lgG is detected in serum. Which of the following is the most likely diagnosis?