Mycology
Core concepts
Fungi are classified as yeasts, molds, and dimorphic fungi. Dimorphic fungi are molds in the environment and yeasts in tissue (temperature-dependent). Know the major pathogens: Candida (germ tube positive for C. albicans), Cryptococcus (encapsulated yeast, India ink, latex agglutination), Aspergillus (acute-angle branching septate hyphae, galactomannan), Mucorales (broad ribbon-like non-septate hyphae, angioinvasive), and the endemic dimorphics (Histoplasma, Blastomyces, Coccidioides, Sporothrix).
Diagnostic logic
Direct microscopy with KOH or calcofluor white, culture, antigen tests (galactomannan, beta-D-glucan, cryptococcal antigen), and molecular assays. Beta-D-glucan is broadly positive in invasive fungal disease but not reliably in mucormycosis or cryptococcosis.
Traps
- Confusing septate (Aspergillus) with non-septate (Mucor) hyphae.
- Assuming beta-D-glucan detects all fungi.
- Forgetting that Cryptococcus antigen is highly sensitive in CSF for meningitis.
- Missing that endemic dimorphics require travel/exposure history.
Worked rule of thumb
A neutropenic patient with fever and a halo-sign nodule on chest CT suggests invasive aspergillosis; send galactomannan and start voriconazole. By contrast, a diabetic with ketoacidosis and a black eschar suggests mucormycosis; treat with liposomal amphotericin B and urgent surgical debridement - azoles alone are inadequate.