A 30-yeаr-оld wоmаn presents with fаtigue, intermittent jоint pain involving both hands, oral ulcers, and a facial rash that worsens after spending time outdoors. She reports increased hair loss over the past several months. Physical examination demonstrates mild swelling of the MCP joints and an erythematous malar rash that spares the nasolabial folds.Which laboratory study is the best initial screening test?