Snapshot A 44-year-old man presents to the emergency department with shortness of breath that has progressively worsened over the course of 6 days. His symptoms are associated with fever, cough, and fatigue. The patient denies any recent travel or sick contacts but is HIV positive and is noncompliant with his medications. The patient has a sulfa-allergy. His temperature is 101°F (38.3°C), blood pressure is 136/88 mmHg, pulse is 89/min, and respirations are 22/min with an oxygen saturation of 91% on room air. Physical examination is notable for crackles in both lungs. A chest radiograph demonstrates bilateral interstitial infiltrates. Laboratory testing is significant for a CD4+ count of 150/uL. He is restarted on his antiretroviral therapy and is started on trimethoprim with dapsone. (Pneumocystis jiroveci pulmonary infection) Introduction Mechanism of action impairs folic acid synthesis via competitive antagonism of para-aminobenzoic acid (PABA) Clinical use leprosy dermatitis herpetiformis pneumocystis jiroveci (serves as prophylaxis treatment) Adverse effects in patients with G6PD deficiency can lead to hemolysis methemoglobinemia