MLT GlobeMycology – page 3
83 Mycology MCQs with answers and explanations.
Which feature is NOT typical of mycetoma (Madura foot)?
Mycetoma is a chronic painless swelling with sinuses discharging grains, caused by fungi or actinomycetes. Systemic fever is not typical.
Which organism does NOT typically cause vaginal discharge?
Cryptococcus causes meningitis and lung disease. The others are associated with vaginosis, vaginitis or cervicitis with discharge.
A gardener gets a nodule after a thorn prick, followed by a chain of nodules along the lymphatics of the arm. The most likely cause is:
Sporothrix schenckii enters via minor trauma and spreads along lymphatics (lymphocutaneous sporotrichosis). Tinea capitis affects the scalp and cryptococcosis is mainly acquired by inhalation.
Narrow, septate hyphae with regular acute-angle (about 45°) branching in lung tissue suggest:
Aspergillus has narrow septate hyphae branching at acute angles. Mucorales have broad, ribbon-like, mostly aseptate hyphae with right-angle branching, which changes treatment.
Calcofluor white is useful for direct fungal detection because it:
Calcofluor white binds beta-linked polysaccharides such as chitin and cellulose in fungal walls and fluoresces blue-white or apple-green depending on filters. A clear halo describes India ink with Cryptococcus.
A yeast isolate from a patient in an ICU is resistant to fluconazole, is misidentified by some biochemical systems, and spreads between patients on surfaces. Which organism is of greatest concern?
Candida auris is often multidrug resistant, persists on surfaces, causes healthcare outbreaks and is misidentified by older biochemical methods. MALDI-TOF with an updated database or molecular methods is recommended.
Which method is recommended for reliable identification of Candida auris?
MALDI-TOF MS with a validated, updated library or DNA sequencing reliably identifies C. auris. It is germ tube negative and gives no characteristic colony on plain SDA, so these cannot identify it.
Which serum or CSF test is most sensitive and specific for diagnosing cryptococcal meningitis?
Cryptococcal antigen (CrAg) lateral flow assay detects capsular glucuronoxylomannan with very high sensitivity. India ink misses many cases, and Cryptococcus has little beta-D-glucan.
Which medium produces brown to black colonies of Cryptococcus neoformans because of phenoloxidase activity?
Niger seed (birdseed) agar contains caffeic acid, which C. neoformans phenoloxidase converts to melanin, giving brown colonies. Cornmeal agar is for morphology, and Mycosel contains cycloheximide, which inhibits Cryptococcus.
Mycosel (Mycobiotic) agar is unsuitable as the only medium for a CSF fungal culture because cycloheximide inhibits:
Cycloheximide inhibits saprophytic molds and several pathogens, including Cryptococcus, some Candida species and Aspergillus. Dermatophytes and dimorphic fungi tolerate it, so a medium without cycloheximide must also be inoculated.
Tissue from a patient with pulmonary disease shows large yeasts (8–15 µm) with thick double-contoured walls and single broad-based buds. The most likely agent is:
Broad-based single budding of large thick-walled yeasts is typical of Blastomyces. Histoplasma yeasts are small (2–4 µm) and intracellular; Paracoccidioides shows multiple narrow buds.
Small oval yeasts, 2–4 µm, packed inside macrophages in a bone marrow smear from an HIV patient suggest:
Histoplasma capsulatum var. capsulatum appears as tiny intracellular yeasts within macrophages. Leishmania amastigotes look similar but have a kinetoplast.
A mold culture at 25 °C shows large, thick-walled tuberculate macroconidia and small microconidia. This suggests:
Tuberculate (knobby) macroconidia are characteristic of the mold phase of Histoplasma capsulatum. Sepedonium species look similar, so conversion or molecular confirmation is needed.
Handling a mold culture of Coccidioides immitis on an open bench is dangerous mainly because it produces:
The mold phase forms alternating barrel-shaped arthroconidia that become airborne and cause laboratory infections. Suspected cultures must be handled in a biosafety cabinet under BSL-3 conditions.
Which dermatophyte genus produces many smooth, thin-walled, club-shaped macroconidia but rarely microconidia, and does NOT infect hair?
Epidermophyton floccosum forms smooth club-shaped macroconidia, often in clusters, with no microconidia, and infects skin and nails but not hair.
Microsporum canis is best recognized in culture by:
M. canis forms large spindle-shaped, rough, thick-walled macroconidia with six or more cells and a yellow colony reverse. Pencil-shaped smooth macroconidia suggest Trichophyton.
A blue-green mold grows rapidly at 45 °C. Microscopy shows uniseriate phialides covering only the upper part of a club-shaped vesicle. The most likely species is:
A. fumigatus has uniseriate phialides on the upper two-thirds of the vesicle, bluish-green colonies and tolerates high temperature. A. niger has black colonies with biseriate phialides covering the whole vesicle.
A corneal scraping from a contact-lens wearer grows a lavender-pink mold with sickle- or canoe-shaped multicelled macroconidia. The genus is:
Fusarium produces banana-shaped multiseptate macroconidia and often pink-purple colonies; it is a major cause of fungal keratitis. Acremonium has only single-celled conidia in slimy heads.
When tissue is submitted for suspected mucormycosis, the laboratory should mince it rather than grind it because:
Pauciseptate hyphae of Mucorales are easily destroyed by homogenization, which lowers culture recovery. Gentle mincing of tissue keeps hyphae viable.
A fast-growing fluffy grey mold fills the Petri dish ('lid lifter') in 2 days. Sporangiophores arise directly above root-like rhizoids. The genus is:
Rhizopus has nodal rhizoids with sporangiophores arising opposite them. Mucor lacks rhizoids, and Lichtheimia has internodal rhizoids with pear-shaped sporangia.