A 32-year-old male with chronic nephritis underwent allogeneic renal transplantation 5 months ago and has been on immunosuppressive therapy. For the past 2 weeks, he has had fever (38.5–39°C), mild cough, and dyspnea. Chest X-ray shows diffuse, fine nodular opacities (∼0.8 mm) uniformly distributed in both lungs, with some areas of coalescence. Hematogenous disseminated pulmonary tuberculosis is suspected. Which test is most valuable for confirmation?