MLT GlobeMycology
83 Mycology MCQs with answers and explanations.
A yeast incubated in serum at 37 °C for 2-3 hours produces short hyphal outgrowths without constriction at the base. This identifies:
A positive germ tube test is the rapid presumptive identification of C. albicans (and C. dubliniensis). C. glabrata and Cryptococcus do not form germ tubes.
A tease mount of a mould in lactophenol cotton blue is examined. This preparation is used to identify:
Lactophenol kills and preserves the fungus while cotton blue stains chitin in hyphae and spores, showing structures for identification.
Sabouraud dextrose agar with low pH, often supplemented with chloramphenicol, is used for fungi. It is classified as a:
Its acidic pH (about 5.6) and added antibiotics inhibit bacteria while allowing fungi to grow, which makes it selective.
Oral thrush (moniliasis) with removable white plaques is caused by:
Moniliasis is an old term for candidiasis; Candida albicans causes oral and vaginal thrush.
Aspergillus niger is used industrially to produce:
A. niger fermentation is the main source of citric acid. Ethanol comes from yeast, lactic acid from Lactobacillus, penicillin from Penicillium.
How do most medically important fungi reproduce and disperse?
Fungi multiply mainly through spores (conidia, sporangiospores, ascospores and others), which are also what laboratories identify microscopically. Cysts, ova and trophozoites belong to protozoa or helminths.
A skin scraping is treated with KOH plus calcofluor white to look for fungal hyphae. Which microscope is needed to read it?
Calcofluor white binds chitin and cellulose and fluoresces blue-white under UV/blue excitation, so a fluorescence microscope is required. It is not visible with ordinary bright-field illumination.
'Ringworm' (tinea) of the skin is caused by which organism?
Ringworm is a dermatophyte fungal infection caused by Microsporum, Trichophyton or Epidermophyton; no worm is involved. Cryptococcus is a yeast causing meningitis.
Which feature is shared by all fungi?
Fungi are eukaryotes whose cell walls contain chitin and glucans; they are heterotrophic (saprophytic or parasitic) and never photosynthetic or worm-like.
Lactophenol cotton blue (LPCB) mounting fluid is routinely used in the laboratory to examine:
LPCB kills, preserves and stains fungal elements (chitin takes up cotton blue), making it the standard mount for mould identification. Protozoa use iodine or trichrome; blood parasites use Giemsa.
Which genus is NOT a dermatophyte?
Dermatophytes comprise Microsporum, Trichophyton and Epidermophyton (plus newer genera like Nannizzia). Madurella causes eumycetoma, a subcutaneous infection.
Moulds that obtain nutrients from dead and decaying organic matter are called:
Saprophytic fungi decompose dead matter; many (e.g. Aspergillus) can become opportunistic pathogens, but their nutritional mode is saprophytic.
A yeast from a vaginal swab is incubated in serum at 37°C for 2–3 hours and produces germ tubes. The organism is most likely:
The germ tube test presumptively identifies C. albicans (and C. dubliniensis). C. glabrata and Cryptococcus are germ tube negative; C. difficile is a bacterium.
Which medium is the standard primary culture medium for isolating fungi from clinical specimens?
SDA has a low pH and high glucose that favour fungi, often with antibiotics added to suppress bacteria.
An encapsulated yeast seen with India ink in CSF of an HIV patient, which is urease positive, is:
Cryptococcus has a polysaccharide capsule that shows as a clear halo in India ink. Cryptococcal antigen testing of CSF or serum is more sensitive than India ink.
Why is 10% potassium hydroxide (KOH) added to skin scrapings before microscopy for fungi?
KOH digests keratin and host cell debris, clearing the specimen so hyphae and yeasts stand out. It does not stain or fluoresce fungi; calcofluor white is the fluorescent reagent.
Which yeast forms in tissue appear as a large cell with multiple buds attached, described as a 'ship's wheel' or 'mariner's wheel'?
Paracoccidioides produces a mother cell with many narrow-necked daughter buds around it. Blastomyces has a single broad-based bud.
A gardener develops nodules that follow the lymphatic vessels up the arm after a thorn injury. Culture at 25 °C shows thin hyphae with conidia arranged like flower petals. The organism is:
Lymphocutaneous spread after a plant injury with a mold showing rosette ('flowerette') conidia is typical of Sporothrix. It converts to cigar-shaped yeasts at 37 °C.
Hair infected with Microsporum canis viewed under a Wood's lamp typically shows:
Most Microsporum species causing ectothrix infection fluoresce yellow-green under Wood's light. Trichophyton tonsurans (endothrix) does not fluoresce; coral-red fluorescence suggests erythrasma.
A mold from an ear swab has a white surface that turns black ('salt and pepper'), with a pale reverse. Microscopy shows large round vesicles fully covered by biseriate phialides. The organism is:
Black conidial heads with a pale reverse and a radiate head covered by biseriate phialides are typical of A. niger, a common cause of otomycosis. Dematiaceous molds such as Alternaria have a dark reverse.