A 23-year-old male with HIV stopped taking all of his medications 3 months ago due to cost. He was feeling fine until 3 weeks ago when he developed a cough. He now has daily fevers (T∼101°F), a nonproductive cough, dyspnea on exertion, fatigue, chills, and tightness in his chest with inspiration. His exam is notable for fever, diaphoresis, bilateral crackles with inspiration, and mild tachypnea. Chest x-ray shows diffuse bilateral interstitial infiltrates. Which of the following is the most likely causative agent for this pulmonary infection?