Q. A 35-year-old female presents to the emergency department with a 2-day history of fever, shortness of breath, and cough. She has a history of HIV infection and has been on antiretroviral therapy (ART) for the past 6 months. The patient also has a history of a Pneumocystis jirovecii lung infection treated 6 months ago. On physical examination, she appears to be in moderate distress, with a temperature of 39.2 degrees Celcius, heart rate of 110/min, and respiratory rate of 28/min. Her oxygen saturation is 90 percent on room air. Lung examination reveals scattered rales and wheezing bilaterally. The remainder of her physical examination is unremarkable. Given her symptoms and history of HIV infection, you suspect an opportunistic infection and order a chest X-ray, blood cultures, and a viral load and CD4 count. Her chest X-ray reveals diffuse interstitial infiltrates, and blood cultures are negative. The viral load is undetectable, and the CD4 count has increased from 120 to 300 cells/mm3 since initiation of ART. The patient is started on empiric antibiotics and corticosteroids. She is admitted to the hospital for further management and observation. Over the next few days, her symptoms gradually improve, and she is eventually discharged on a tapering course of prednisone. Which of the following is the most likely cause of her acute illness?