A 33-year-old woman has had fever and increasing fatigue for the past 2 months. Over the past year, she has noticed the soreness of her muscles and joints and has had a 4-kg weight loss. On physical examination, her temperature is 37.5degC, pulse is 80/min, respirations are 15/min, and blood pressure is 145/95 mm Hg. She has pain on deep inspiration, and a friction rub is heard on auscultation of the chest. Laboratory findings show glucose, 73 mg/dL; total protein, 5.2 g/dL; albumin, 2.9 g/dL; and creatinine, 2.4 mg/dL. Serum complement levels are decreased. CBC shows hemoglobin of 9.7 g /dL, platelet count of 85,000/mm3, and WBC count of 3560/mm3. A renal biopsy specimen shows diffuse proliferative glomerulonephritis with extensive granular immune deposits of IgG and C1q in capillary loops and mesangium. After being treated with immunosuppressive therapy consisting of prednisone and cyclophosphamide, her condition improves. Which of the following serologic studies is most likely to be positive in this patient?