In neutropenic patients, which organism is most commonly associated with invasive pulmonary infection?

• A. Aspergillus fumigatus
• B. Histoplasma capsulatum
• C. Cryptococcus neoformans
• D. Candida albicans

Answer: (A)

In neutropenic patients, the lungs are most commonly invaded by Aspergillus fumigatus. The lack of neutrophils removes the main defense against hyphal fungi, allowing inhaled Aspergillus hyphae to penetrate alveolar walls and blood vessels, causing necrotizing, angioinvasive pneumonia that can spread beyond the lungs. Clinically this often presents with fever and respiratory symptoms, and imaging may show pulmonary nodules with a halo sign due to surrounding hemorrhage. The organism is a mold with septate hyphae that branch at acute angles, and positive galactomannan testing can support the diagnosis.

Histoplasma capsulatum can cause severe disease in immunocompromised hosts but tends to involve intracellular yeast within macrophages and is not the classic angioinvasive pulmonary infection seen with neutropenia. Cryptococcus neoformans is more associated with meningitis in severely immunocompromised patients and less typically causes the primary invasive pulmonary infection in this setting. Candida albicans commonly causes candidemia and disseminated infection rather than a focal invasive pulmonary process in neutropenic patients.