A 52-year-old woman is evaluated for a 6-year history of Sjögren syndrome. During the past 3 months, she has had low-grade fevers up to 37.5 °C (99.5 °F), weight loss of 6.8 kg (15 lb), and increased fatigue and sicca symptoms. She recently noted a rash on her legs. She reports no current joint pain. Medications are hydroxychloroquine and acetaminophen as needed.
On physical examination, temperature is 37.2 °C (99.0 °F), blood pressure is 135/85 mm Hg, and pulse rate is 82/min. BMI is 28. The oral mucosa is dry. Bilateral parotid fullness is present. There is bilateral cervical adenopathy. The tip of the spleen is palpable. There are a few scattered palpable purpura on the lower legs. The remainder of the physical examination is unremarkable.
Laboratory studies show a normal complete blood count except for a hemoglobin level of 11 g/dL (110 g/L); serum C3 and C4 levels are low, and serum and urine protein electrophoresis reveals M-component.
Chest radiograph and echocardiogram are normal. CT scan of the abdomen shows numerous enlarged retroperitoneal lymph nodes and splenomegaly.
Which of the following is the most appropriate next step in the management of this patient?