A 38-year-old man with presents to the emergency department with a two-week history of progressive shortness of breath, non-productive cough, and fever. He has a history of human immunodeficiency virus (HIV) and reports not taking his antiretroviral therapy for the past 4 months. His last CD4 count was 180/mm3. His medical history is otherwise unremarkable, and he does not take any medications. Vital signs are temperature 99.9°F (37.7°C), pulse 108 beats/min, respirations 23/min, and oxygen saturation 94% on room air. On physical examination, he is tachypneic and has diffuse crackles on lung auscultation. A chest X-ray reveals bilateral interstitial infiltrates.
Which of the following is the most likely causative organism?
The correct answer is:
D) Pneumocystis jirovecii
HIV-positive patients with CD4 counts less than 200/mm3 are susceptible to Pneumocystis jirovecii pneumonia (PCP). This organism typically causes bilateral interstitial lung infiltrates and can be seen with silver stain of a bronchoalveolar lavage. The patient's symptoms, CD4 count, and chest X-ray findings are consistent with PCP.
Answer choice A: Aspergillus fumigatus, is incorrect. Aspergillus typically causes invasive pulmonary aspergillosis in neutropenic patients and presents with cavitary lesions rather than interstitial infiltrates.
Answer choice B: Cytomegalovirus, is incorrect. Cytomegalovirus can cause pneumonia in immunocompromised patients, but it is less common than PCP in patients with low CD4 counts and typically presents with more diffuse symptoms.
Answer choice C: Mycobacterium tuberculosis, is incorrect. Mycobacterium tuberculosis can cause pulmonary symptoms in patients with HIV infection, but it typically presents with upper lobe cavitary lesions rather than interstitial infiltrates.
Answer choice E: Streptococcus pneumoniae, is incorrect. Streptococcus pneumoniae typically causes lobar pneumonia with consolidation rather than interstitial infiltrates.
Key Learning Point
HIV-positive patients with CD4 counts less than 200/mm3 are susceptible to Pneumocystis jirovecii pneumonia (PCP). This organism typically causes bilateral interstitial lung infiltrates and can be seen with silver stain of a bronchoalveolar lavage.
