Pneumocystis jirovecii (Pneumocystis pneumonia)

A 58-year-old man goes to the emergency department for evaluation of fever and dry cough for the past week. He has also become progressively more dyspneic, and walking a single block makes him short of breath. He underwent a liver transplantation 4-months ago for treatment of autoimmune hepatitis and is currently taking several immunosuppressant drugs. Temperature is 38.5°C (101.3°F), pulse is 104/min, respirations are 20/min, blood pressure is 122/88 mmHg, and oxygen saturation is 89% on room air. Lung auscultation reveals bilateral crackles and rhonchi. Chest radiograph reveals bilateral, diffuse interstitial infiltrates extending from the perihilar region. Leukocyte count is 13,000/mm3 and serum lactate dehydrogenase level is 460 U/L. Further testing is most likely to reveal which of the following findings?