SM Mycology & Fungal Diagnostics 1 — Questions and Answers
Question 1: Which stain is most commonly used for the direct microscopic detection of fungi in clinical specimens due to its ability to bind chitin?
- Gram stain
- Calcofluor white (Correct answer)
- Ziehl-Neelsen stain
- Giemsa stain
Correct answer: Calcofluor white
Calcofluor white binds to chitin in fungal cell walls and fluoresces brightly under UV light, making it highly sensitive for detecting fungi directly in clinical specimens.
Question 2: Which Candida species produces a germ tube when incubated in serum at 37°C for 2–3 hours?
- Candida glabrata
- Candida tropicalis
- Candida albicans (Correct answer)
- Candida krusei
Correct answer: Candida albicans
Candida albicans (and C. dubliniensis) produces germ tubes — true hyphal extensions without a constriction at the origin — when incubated in serum at 37°C, enabling rapid presumptive identification.
Question 3: Which preparation is used to visualize the characteristic polysaccharide capsule of Cryptococcus neoformans in CSF?
- KOH preparation
- India ink preparation (Correct answer)
- Lactophenol cotton blue mount
- Gram stain
Correct answer: India ink preparation
India ink preparation creates a negative stain in which the large polysaccharide capsule of Cryptococcus neoformans appears as a clear halo surrounding the yeast cell against the dark background.
Question 4: Which culture medium is the standard primary isolation medium for fungi in clinical laboratories due to its selective acidic pH?
- Blood agar
- Thayer-Martin agar
- Sabouraud dextrose agar (Correct answer)
- Chocolate agar
Correct answer: Sabouraud dextrose agar
Sabouraud dextrose agar (pH 5.6) selectively supports fungal growth while inhibiting most bacteria due to its acidic pH and high glucose concentration.
Question 5: Histoplasma capsulatum, a thermally dimorphic fungus, converts to which morphological form at 37°C (human body temperature)?
- Hyaline septate hyphae
- Yeast cells (Correct answer)
- Spherules containing endospores
- Arthrospores
Correct answer: Yeast cells
Thermally dimorphic fungi such as Histoplasma capsulatum convert from mold (room temperature, 25–30°C) to yeast form at body temperature (37°C), which is the tissue-invasive phase.
Question 6: What is the primary virulence factor of Cryptococcus neoformans that enables it to evade phagocytosis?
- Germ tube formation
- Beta-glucan in cell wall
- Polysaccharide capsule (Correct answer)
- Protease enzyme secretion
Correct answer: Polysaccharide capsule
The polysaccharide capsule (composed mainly of glucuronoxylomannan) is the primary virulence factor of Cryptococcus neoformans, inhibiting phagocytosis, complement activation, and antigen presentation.
Question 7: Dermatophytes are distinguished from other fungi by their primary ability to infect which tissue types?
- Internal organs and bloodstream
- CNS and meninges
- Keratinized tissues including skin, hair, and nails (Correct answer)
- Mucous membranes and visceral organs
Correct answer: Keratinized tissues including skin, hair, and nails
Dermatophytes (Trichophyton, Microsporum, Epidermophyton) produce keratinases enabling colonization and infection of keratinized structures such as skin, hair, and nails, but they do not invade deeper tissues.
SM Mycology & Fungal Diagnostics 1
Which stain is most commonly used for the direct microscopic detection of fungi in clinical specimens due to its ability to bind chitin?