A 28-year-old woman presents with a 6-week history of arthralgia, photosensitive malar rash, and oral ulcers. She also reports recent-onset dyspnea and pleuritic chest pain. Physical examination reveals a butterfly rash, mild fever, and bilateral pleural effusions on auscultation. Serum ANA is positive at 1:640 with a homogeneous pattern. Which investigation is most appropriate to confirm the diagnosis of SLE and assess for lupus nephritis?