A 27-year-old female presents to the emergency room with a temperature of 103degF, severe fatigue, weight loss, and joint pain. During the history and physical examination, the patient reports that she stopped taking her aspirin and corticosteroids to control her condition. A butterfly-type rash over her cheeks, sensitivity to light, and a heart murmur are apparent. The patient also reports a history of a progressively developing arthritis and glomerulonephritis. Laboratory tests further indicate anemia, leukopenia, and thrombocytopenia. This condition is best diagnosed by the presence of which of the following?