An 18-year-old man presents to the clinic with a complaint of fatigue and a cough productive of green sputum for the past two days. He has no significant medical conditions. Social history reveals that he is sexually active with other men. Vital signs show a temperature of 101°F (38.3°C), heart rate 96 beats/minute, respirations 21/minute, and oxygen saturation 98% on room air. Physical examination reveals tachycardia, dyspnea, and bilateral crackles on auscultation of the lungs. Blood tests show increased levels of lactate dehydrogenase and a CD4 count 182/μL. HIV test is ordered but is pending. Chest x-ray reveals diffuse, bilateral, interstitial infiltrates. He is prescribed trimethoprim-sulfamethoxazole.
Which of the following is the most likely causative organism?
- A) Haemophilus influenzae
- B) Pneumocystis jirovecii
- C) Pseudomonas aeruginosa
- D) Staphylococcus aureus
- E) Streptococcus pyogenes
B) Pneumocystis jirovecii
Pneumocystis jirovecii is a fungus that can cause a lethal pneumonia in hosts with debilitated immune systems. The history and physical examination suggest pneumonia, and the CD4 count under 200/μL in a patient with HIV infection is diagnostic of acquired immunodeficiency syndrome (AIDS). An elevated lactate dehydrogenase can be used as a clinical indicator of possible Pneumocystis (PCP) in HIV-infected individuals but has little utility in patients without HIV infection. The typical chest x-ray findings of PCP are diffuse, bilateral, interstitial infiltrates. Although all of the answer choices listed can cause pneumonia, the clinical findings outlined above are most consistent with Pneumocystis jirovecii. Trimethoprim-sulfamethoxazole (TMP-SMX) is the treatment of choice for Pneumocystis pneumonia (PCP).
Key Learning Point
Pneumocystis jirovecii can cause pneumonia in immunocompromised individuals, and those with HIV infection with low CD4 count under 200/μL are at highest risk of PCP. Trimethoprim-sulfamethoxazole is the preferred treatment for PCP. Those with abnormalities in oxygen exchange at the time of presentation benefit from corticosteroids as well.
