SM Mycology & Fungal Diagnostics 2 — Questions and Answers
Question 1: Which Aspergillus species accounts for approximately 90% of invasive aspergillosis cases in immunocompromised patients?
- Aspergillus niger
- Aspergillus flavus
- Aspergillus fumigatus (Correct answer)
- Aspergillus terreus
Correct answer: Aspergillus fumigatus
Aspergillus fumigatus is the predominant cause of invasive aspergillosis due to its thermotolerance at 37°C, small conidial size enabling deep pulmonary penetration, and abundant environmental distribution.
Question 2: The serum galactomannan enzyme immunoassay (EIA) is an FDA-approved test for diagnosing invasive infection caused by which fungal genus?
- Candida
- Cryptococcus
- Aspergillus (Correct answer)
- Mucor
Correct answer: Aspergillus
Galactomannan is a cell wall polysaccharide shed by Aspergillus species during active hyphal growth; its detection in serum or BAL is an FDA-approved biomarker for invasive aspergillosis.
Question 3: Blastomyces dermatitidis infection is most commonly acquired through which route?
- Direct person-to-person contact
- Inhalation of conidia from contaminated soil (Correct answer)
- Ingestion of contaminated water
- Skin inoculation via animal bites
Correct answer: Inhalation of conidia from contaminated soil
Blastomyces dermatitidis exists as a mold in moist soil and decaying organic matter; infection occurs primarily through inhalation of aerosolized conidia from disturbed environmental sources.
Question 4: Which microscopic finding is most characteristic of Mucormycetes (zygomycetes) in tissue sections or wet preparations?
- Narrow, acute-angle, septate hyphae (45°)
- Broad, pauciseptate (rarely septate), ribbon-like hyphae with right-angle branching (Correct answer)
- Spherules containing endospores
- Encapsulated budding yeast cells
Correct answer: Broad, pauciseptate (rarely septate), ribbon-like hyphae with right-angle branching
Mucorales (Mucor, Rhizopus, Lichtheimia) characteristically produce broad (10–20 µm), ribbon-like, pauciseptate hyphae that branch at right angles, distinguishing them from the narrow, septate, acute-angle hyphae of Aspergillus.
Question 5: Pneumocystis jirovecii cysts in bronchoalveolar lavage (BAL) specimens are best visualized using which staining method?
- Gram stain
- Gomori methenamine silver (GMS) or calcofluor white (Correct answer)
- India ink preparation
- Giemsa stain alone
Correct answer: Gomori methenamine silver (GMS) or calcofluor white
GMS stain (gold standard) and calcofluor white stain the cyst walls of Pneumocystis jirovecii black and fluorescent respectively, enabling identification in BAL specimens; Giemsa stains trophic forms but not cyst walls.
Question 6: Azole antifungal agents (fluconazole, voriconazole, itraconazole) exert their primary antifungal effect by inhibiting which enzyme?
- Echinocandin-sensitive glucan synthase
- Ergosterol binding at the cell membrane
- Lanosterol 14-α-demethylase (CYP51) (Correct answer)
- Chitin synthase in the cell wall
Correct answer: Lanosterol 14-α-demethylase (CYP51)
Azoles inhibit the fungal cytochrome P450 enzyme lanosterol 14-α-demethylase (CYP51), blocking ergosterol biosynthesis and leading to accumulation of toxic methylated sterols in the fungal membrane.
Question 7: Sporothrix schenckii infection (sporotrichosis) is most classically acquired through which mechanism?
- Inhalation of airborne spores from decaying vegetation
- Traumatic inoculation via rose thorns or sphagnum moss (Correct answer)
- Ingestion of contaminated soil particles
- Sexual transmission from infected individuals
Correct answer: Traumatic inoculation via rose thorns or sphagnum moss
Sporotrichosis ('rose thorn disease') is a subcutaneous mycosis classically acquired through traumatic inoculation with Sporothrix schenckii-contaminated plant material such as rose thorns, sphagnum moss, or wood splinters.
SM Mycology & Fungal Diagnostics 2
Which Aspergillus species accounts for approximately 90% of invasive aspergillosis cases in immunocompromised patients?