A 21-year-old woman presents with a 3-month history of malaise, joint pain, weight loss, and sporadic fever. The patient appears agitated. Her temperature is 38degC (101degF). Other physical findings include malar rash, erythematous pink plaques with telangiectatic vessels, oral ulcers, and non blanching purpuric papules on her legs. Laboratory studies show elevated levels of blood urea nitrogen and creatinine. Antibodies directed to which of the following antigens would be expected in the serum of this patient?