A 36-year-old man with AIDS presents to the emergency department with 10 days of a dry, nonproductive cough, intermittent fevers, malaise, and decreased appetite. Over the past 2 days he has developed...

A 36-year-old man with AIDS presents to the emergency department with 10 days of a dry, nonproductive cough, intermittent fevers, malaise, and decreased appetite. Over the past 2 days he has developed difficulty breathing and severe dyspnea on exertion. His vital signs are significant for mild tachypnea and an oxygen saturation of 88% on room air. He has oral thrush and his pulmonary exam reveals bilateral fine rales. A chest x-ray shows bilateral perihilar interstitial opacities. What is the first-line antibiotic agent for this condition?

A. Penicillin G

B. Trimethoprim/sulfamethoxazole

C. Azithromycin

D. Doxycycline

E. Rifampin