In the context of opportunistic infections in HIV, which organism is most typically implicated?

• A. Histoplasma capsulatum
• B. Candida albicans
• C. Mycobacterium tuberculosis
• D. Pneumocystis jiroveci

Answer: (D)

When the immune system is severely weakened in HIV, the most typical opportunistic pneumonia is caused by Pneumocystis jirovecii. This organism is a fungal pathogen that takes advantage of low CD4 counts (especially below 200) to cause Pneumocystis jirovecii pneumonia, a classic opportunistic infection in AIDS. It commonly presents with a subacute onset of progressive shortness of breath, a dry cough, fever, and hypoxemia, and shows diffuse interstitial infiltrates on imaging. Diagnosis relies on detecting the organism in respiratory specimens with special stains or immunofluorescence, since it’s not easily cultured. Clinically, it’s the prototypical HIV-related opportunistic pulmonary infection, which is why it’s the best answer here. The other organisms listed can cause disease in HIV, but they do not define the most common opportunistic pneumonia seen in these patients.