Which organism commonly causes oral thrush in immunocompromised patients?

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

Answer: (B)

Oral thrush in people with weakened immune systems is most commonly caused by Candida albicans, a yeast that normally resides in the mouth and other mucosal sites without causing disease. When immune defenses are down or the normal flora is disrupted, this organism can overgrow and invade the mucosa, producing the characteristic creamy white plaques that can be scraped away to reveal a red, inflamed surface. Candida albicans is adept at sticking to mucosal surfaces and forming biofilms in the moist environment of the mouth, and it can be identified in the lab as a budding yeast that often shows pseudohyphae; the germ tube test is a classic positive for this species.

The other fungi listed cause different clinical problems: Aspergillus fumigatus typically leads to invasive lung disease in severely immunocompromised patients; Cryptococcus neoformans is a common cause of meningitis in the immunosuppressed; Histoplasma capsulatum causes histoplasmosis, which can disseminate in those with weakened immunity. None of these primarily present as oral thrush, which is why Candida albicans is the best answer.