A 45-year-old male presents to the emergency depament with 2 days of haemoptysis. He repos that he has been coughing up 1-3 tablespoons of blood each day. He does repo mild chest pain, low-grade fevers, and weight loss. He has had about 1 year of severe upper respiratory symptoms including frequent epistaxis and purulent discharge treated with several courses of antibiotics. His only medications are daily aspirin and lovastatin. On examination, he has normal vital signs, and upper airway is notable for saddle nose deformity and clear lungs. A CT scan of the chest shows multiple cavitating nodules, and urinalysis shows RBCs . Which of the following tests offers the highest diagnostic yield to make the appropriate diagnosis?