MLT GlobeMycology – page 4
83 Mycology MCQs with answers and explanations.
Skin biopsy from a farm worker with chronic warty lesions of the leg shows brown, thick-walled, round cells with cross-walls ('copper pennies'). The diagnosis is:
Pigmented muriform (sclerotic) bodies in tissue define chromoblastomycosis, caused by agents such as Fonsecaea pedrosoi. Phaeohyphomycosis shows pigmented hyphae, and mycetoma shows grains.
A dematiaceous mold shows brown, club-shaped conidia with both transverse and longitudinal septa, formed in chains. The genus is:
Alternaria forms drumstick-shaped muriform conidia in chains. Curvularia conidia are curved with transverse septa only, and Cladosporium produces small shield-shaped conidia.
Which Candida species is intrinsically resistant to fluconazole and should be reported with this comment?
Pichia kudriavzevii (C. krusei) is intrinsically resistant to fluconazole. The other species are usually susceptible, although acquired resistance can occur.
Serum galactomannan testing is mainly used to support the diagnosis of:
Galactomannan is a cell wall antigen of Aspergillus, used in serum or bronchoalveolar lavage from high-risk haematology patients. Mucorales do not release galactomannan.
Which laboratory test is most useful for rapid diagnosis of disseminated histoplasmosis in an immunocompromised patient?
Histoplasma polysaccharide antigen in urine (and serum) is highly sensitive in disseminated disease. Galactomannan can cross-react but is not the test of choice.
A child has scaly patches of hair loss with 'black dots' where hairs broke at the scalp. Wood's lamp shows no fluorescence, and spores fill the inside of the hair shaft. The most likely dermatophyte is:
T. tonsurans causes endothrix 'black dot' tinea capitis and does not fluoresce. Microsporum canis and M. audouinii are ectothrix and give green fluorescence.
On CHROMagar Candida, Candida albicans typically forms colonies that are:
The chromogenic medium gives green C. albicans colonies; C. tropicalis is blue and C. krusei pink and rough.
Histoplasma capsulatum is classically acquired by inhaling spores from soil contaminated with:
Histoplasmosis ('cave disease') follows inhalation of microconidia from soil rich in bat or bird guano.
In tissue, Coccidioides immitis is seen as:
Coccidioides forms thick-walled spherules containing endospores in tissue, and arthroconidia in culture.
A dimorphic mould from an HIV patient from South-East Asia produces a diffusible red pigment at 25 °C. The most likely organism is:
T. marneffei is the only dimorphic 'Penicillium', produces a red diffusible pigment and causes disseminated infection in HIV patients in South-East Asia.
Which dermatophyte is the most common cause of tinea pedis and fungal nail infections worldwide?
T. rubrum is the leading cause of chronic foot and nail infection; M. canis mainly causes scalp and body ringworm from pets.
On dermatophyte test medium (DTM), a dermatophyte is suggested by:
Dermatophytes use protein and make the medium alkaline, turning the phenol red indicator red.
In lactophenol cotton blue mounting fluid, the phenol component mainly:
Phenol kills the organism, lactic acid preserves structures and cotton blue stains the fungal wall.
Lactophenol cotton blue mount of a mould from a BAL sample shows columnar conidial heads with phialides only on the upper part of the vesicle. Likely identity?
A. fumigatus has uniseriate phialides on the upper two-thirds of the vesicle, forming columnar heads; it is the commonest cause of invasive aspergillosis. A. niger has biseriate, radiate black heads; Rhizopus has sporangia.
Candida auris differs from Candida albicans in that C. auris typically:
C. auris grows well at 40–42 °C, does not produce germ tubes, and rarely forms hyphae or chlamydospores. Chlamydospore production on cornmeal agar is characteristic of C. albicans.
A dimorphic fungus grows as a mold at 25 °C and as a yeast at 37 °C. Which organism is NOT dimorphic in this way?
In tissue Coccidioides forms spherules containing endospores, not yeast cells. Histoplasma, Blastomyces and Talaromyces all show classic mold-to-yeast thermal dimorphism.
A dermatophyte isolate produces teardrop microconidia along the hyphae, a wine-red colony reverse on potato dextrose agar, and is urease negative. The most likely species is:
T. rubrum typically gives a red reverse on potato dextrose agar, teardrop microconidia and a negative urease at 7 days. T. interdigitale (mentagrophytes complex) is usually urease positive within a few days.
The in vitro hair perforation test is used to separate:
T. mentagrophytes complex forms wedge-shaped perforations in hair in vitro, whereas T. rubrum does not. It is a classic test for these two look-alike dermatophytes.
A black, yeast-like colony that later becomes fuzzy shows annellides producing clusters of oval conidia at their tips. This dematiaceous organism is most likely:
Exophiala (black yeast) begins yeast-like and forms tapered annellides with conidia collecting at the tip. Alternaria is a rapidly growing woolly mold with large muriform conidia.
A yeast from urine forms small cells only, with no pseudohyphae on cornmeal agar, and rapidly hydrolyses trehalose. The most likely identification is:
C. glabrata forms small blastoconidia without pseudohyphae and gives a rapid trehalose test. C. albicans and C. tropicalis form pseudohyphae; C. krusei forms elongated 'crossed-matchstick' pseudohyphae.