A 10-year-old girl complains of persistent redness of the skin over her knuckles and around the nail beds. She describes easy fatigability and can rise only with difficulty from a squatting position. Physical examination reveals erythema over the knuckles and a heliotropic rash. A muscle biopsy shows infiltrates of B and T lymphocytes around blood vessels and in connective tissue of the perimysium. Elevated serum levels of which of the following would be expected in this patient?