A 52-year-old man is admitted to the hospital with a 3-week history of fever, cough, and exertional dyspnea. Oxygen saturation is 92% on room air, and chest radiography shows bilateral, diffuse, hazy opacities (Figure 1). An arterial blood gas demonstrates partial pressure of arterial oxygen (PaO2) of 65 mmHg and alveolar-arterial gradient (A-a gradient) of 40 mmHg. An HIV-1/2 antigen-antibody immunoassay is positive and empiric therapy for Pneumocystis pneumonia is initiated with intravenous trimethoprim-sulfamethoxazole. Additional laboratory studies show a serum lactate dehydrogenase (LDH) of 650 IU/L and plasma beta-D-glucan of 620 pg/mL.