A 24-year-old woman is referred to the clinic for assessment of a low white-cell count. She has no past medical history and is not on any medications. Her only symptoms are of joint discomfort in her hands, and occasional sharp chest pains that change with breathing. On physical examination, there is inflammation of some MCP and DIP joints in both hands, and the rest of the examination is normal. Her WBC is 3500/mL and on the differential the lymphocytes are low (15%) and PMNs are normal. Which of the following is the most likely diagnosis?