A 28-year-old woman is evaluated in the emergency department for a 1-day history of progressive shortness of breath, cough, and hemoptysis. She reports a fever but no chills. She has a 2-year history of systemic lupus erythematosus. Medications are mycophenolate mofetil, hydroxychloroquine, prednisone, naproxen as needed, vitamin D, and calcium.
On physical examination, temperature is 38.9 °C (102.0 °F), blood pressure is 100/60 mm Hg, pulse rate is 110/min, and respiration rate is 24/min. Oxygen saturation is 88% on ambient air. BMI is 29. Diffuse hair thinning is noted. A malar rash is present. There is symmetric swelling of metacarpophalangeal and proximal interphalangeal joints as well as both wrists and knees. There are no cardiac rubs or murmurs. Diffuse crackles are heard on lung auscultation.
Hematocrit | 22% (30% in office 1 week ago) |
Leukocyte count | 3200/µL (3.2 × 109/L) |
Platelet count | 90,000/µL (90 × 109/L) |
Creatinine | 1.3 mg/dL (115 µmol/L) |
Urinalysis | 3+ blood; 2+ protein; erythrocyte casts |
Chest radiograph reveals diffuse bilateral pulmonary infiltrates with sparing of the apices.
Which of the following is most likely to establish a diagnosis?