Mycology: specimens, microscopy and culture

36 Kerala PSC Lab Technician Gr II practice questions on Mycology: specimens, microscopy and culture, part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Medium

   A specimen from a suspected dimorphic fungal infection is cultured on two sets of media, one held at 25°C and one at 37°C, in order to:

   1. ASuppress contaminating bacteria
   2. BPreserve the medium from drying out
   3. CSpeed up sporulation of the mould
   4. DShow the mould-to-yeast conversion

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   Answer: D · Show the mould-to-yeast conversion

   Dimorphic fungi such as Histoplasma capsulatum and Sporothrix schenckii grow as a mould at room temperature and as a yeast at body temperature. Growing the same isolate at both temperatures and showing the conversion is a confirmatory step in their identification. Conversion can take several weeks, so molecular or antigen tests are used when a fast answer is needed. All work on these cultures is done in a biosafety cabinet.

   • A: Bacteria are suppressed by chloramphenicol in the medium.
   • B: Drying is prevented by screw-capped tubes and a humid incubator.
   • C: Sporulation is governed by the medium and the fungus, not by paired temperatures.

2. Question 2 · Medium

   Sputum for fungal culture should be an early morning specimen because it:

   1. AHolds secretions pooled overnight
   2. BCan be stored for longer
   3. CNeeds no transport medium
   4. DContains fewer oral yeasts

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   Answer: A · Holds secretions pooled overnight

   Secretions collected in the airways through the night give the richest sample, so the first specimen after waking has the best yield. The patient rinses the mouth with water first and coughs deeply into a sterile wide-mouthed container. A 24-hour collection is not accepted because it overgrows with bacteria and oral yeasts. Three specimens on consecutive days are usual when a fungal pneumonia is suspected.

   • B: Storage time depends on refrigeration, not on the hour of collection.
   • C: Sputum needs no transport medium whatever time it is taken.
   • D: Oral yeasts are present at any time of day; rinsing the mouth reduces them.

3. Question 3 · Easy

   At about what pH is Sabouraud dextrose agar prepared?

   1. A7.2
   2. B4.0
   3. C8.4
   4. D5.6

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   Answer: D · 5.6

   Sabouraud dextrose agar contains 4% glucose and peptone at a slightly acid pH, close to 5.6, which favours fungi and discourages most bacteria. Chloramphenicol and cycloheximide are added when a heavily contaminated specimen such as skin or sputum is cultured. Routine bacteriological media are kept near neutral pH 7.2 to 7.4. Tubes are incubated at room temperature and at 37°C, and held for up to 4 weeks.

   • A: pH 7.2 is the range used for routine bacteriological media.
   • B: pH 4.0 is more acid than the medium is prepared at.
   • C: An alkaline pH of 8.4 would not favour fungal growth.

4. Question 4 · Hard

   A yeast from the blood culture of a patient on broad-spectrum antibiotics is germ tube negative, urease negative and shows no capsule on India ink. Corn meal agar shows only small budding cells, with no pseudohyphae or chlamydospores. The likely identification is:

   1. ACandida tropicalis
   2. BCandida glabrata
   3. CCandida albicans
   4. DCryptococcus neoformans

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   Answer: B · Candida glabrata

   Candida glabrata is the one common Candida species that makes neither pseudohyphae nor chlamydospores, so corn meal agar shows only blastoconidia. The negative germ tube test excludes C. albicans, and the negative urease and India ink results exclude Cryptococcus. C. tropicalis forms abundant pseudohyphae with blastoconidia along them. The species is now named Nakaseomyces glabratus, and it is often less susceptible to fluconazole, so sensitivity testing is requested.

   • A: Candida tropicalis forms pseudohyphae with blastoconidia on corn meal agar.
   • C: Candida albicans is germ tube positive and forms chlamydospores.
   • D: Cryptococcus neoformans is urease positive and capsulated.

5. Question 5 · Hard

   A KOH mount of skin scrapings shows branching septate hyphae, but Sabouraud dextrose agar with chloramphenicol and cycloheximide is sterile after 4 weeks at 25°C. The most likely explanation is:

   1. AAn antifungal cream was used before sampling
   2. BCycloheximide inhibited the dermatophyte
   3. CThe hyphae seen were cotton fibres
   4. DThe plates were read too early

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   Answer: A · An antifungal cream was used before sampling

   Topical antifungal treatment kills the fungus in the upper scales while its dead hyphae can still be seen on KOH for days, so microscopy stays positive and culture fails. Patients are therefore asked to stop creams for about a week before scrapings are repeated. Cotton or other fibres have no septa or branching and do not taper, so they are not mistaken for hyphae by a careful reader. Dermatophytes are resistant to cycloheximide, and 4 weeks is the full incubation period.

   • B: Dermatophytes are resistant to cycloheximide; it suppresses saprophytes.
   • C: Fibres lack septa and branching, and the mount showed both.
   • D: Four weeks is the standard period before a fungal culture is reported negative.

6. Question 6 · Easy

   How are skin scrapings and nail clippings for fungal study best sent to the laboratory?

   1. APeptone water
   2. BA folded paper packet
   3. CFormalin, 10%
   4. DA tube of normal saline

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7. Question 7 · Easy

   A yeast from a blood culture is suspended in human serum and held at 37°C for 2 hours. Short filaments arise from the cells without constriction at their origin. This test is the:

   1. AUrease test
   2. BNiger seed test
   3. CGerm tube test
   4. DSlide culture

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8. Question 8 · Medium

   Scrapings for a suspected ringworm lesion are taken from the raised edge of the patch because:

   1. AThe edge is least painful to scrape
   2. BThe centre of the patch holds more keratin
   3. CLiving hyphae are most numerous there
   4. DBacteria do not grow at the edge

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9. Question 9 · Easy

   On which medium are chlamydospores of Candida albicans demonstrated?

   1. AThayer-Martin agar
   2. BMacConkey agar
   3. CLoeffler serum slope
   4. DCorn meal agar with Tween 80

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10. Question 10 · Medium

    Why are plates and tubes of a mould culture examined only inside a biological safety cabinet?

    1. AMoulds grow only in a laminar airflow
    2. BRoom air contaminates the culture at once
    3. CConidia are easily aerosolised and inhaled
    4. DLight outside the cabinet kills the fungus

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11. Question 11 · Medium

    A germ tube test was set up in serum at 37°C and read the next morning. Filaments were seen, but the yeast was later identified as Candida tropicalis. What was the error?

    1. AIncubating at 37°C instead of at 25°C
    2. BUsing too few yeast cells
    3. CUsing serum instead of saline
    4. DReading the test after too long

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12. Question 12 · Medium

    Why is a slide culture (Riddell's method) preferred to a tease mount for identifying a mould?

    1. AIt kills the mould before it is examined
    2. BIt makes the mould grow much faster
    3. CIt keeps spores in their natural arrangement
    4. DIt shows the true colour of the colony reverse

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13. Question 13 · Medium

    On CHROMagar Candida, a yeast that forms green colonies is most likely:

    1. APichia kudriavzevii (C. krusei)
    2. BCandida parapsilosis
    3. CCandida albicans
    4. DCandida tropicalis

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14. Question 14 · Medium

    A 7-year-old boy has scaly patches of hair loss with broken-off hairs. How should hair be collected for fungal study?

    1. APlucked with forceps, with the root
    2. BPlaced in formalin for transport
    3. CCollected from his pillow, cap or comb
    4. DCut with scissors near the tips

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15. Question 15 · Hard

    Scrapings from scaly, pale patches on a young man's upper back show short curved hyphae with clusters of round yeast cells in KOH. Plain Sabouraud dextrose agar shows no growth after a week. What should the laboratory do to culture the organism?

    1. AOverlay the agar with sterile olive oil
    2. BUse dermatophyte test medium
    3. CAdd cycloheximide to the medium
    4. DIncubate the plate at 42°C

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16. Question 16 · Easy

    In lactophenol cotton blue, which ingredient kills the fungus?

    1. ALactic acid
    2. BCotton blue
    3. CGlycerol
    4. DPhenol

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17. Question 17 · Easy

    In a KOH mount of skin or nail, potassium hydroxide is used to:

    1. AFix the fungus to the slide
    2. BDigest keratin and clear the tissue
    3. CStain the fungal cell wall a deep blue
    4. DKill bacteria in the sample

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18. Question 18 · Easy

    Which antibiotic is added to Sabouraud dextrose agar to suppress fast-growing saprophytic moulds?

    1. AGentamicin
    2. BChloramphenicol
    3. CPenicillin
    4. DCycloheximide

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19. Question 19 · Hard

    CSF from a patient with AIDS shows capsulated budding yeasts on India ink. It was plated only on Sabouraud dextrose agar with chloramphenicol and cycloheximide at 25°C, and there is no growth after 7 days. The most likely reason is:

    1. AChloramphenicol in the medium inhibited it
    2. BCycloheximide in the medium inhibited it
    3. CCryptococcus needs 37°C to grow at all
    4. DThe India ink killed the yeasts in the CSF

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20. Question 20 · Easy

    An India ink (nigrosin) preparation of CSF is a negative stain used to show the:

    1. ASpores of Rhizopus species
    2. BGerm tubes of Candida albicans
    3. CCapsule of Cryptococcus neoformans
    4. DBranching hyphae of Aspergillus species

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21. Question 21 · Easy

    In a Gram-stained smear, Candida albicans appears as:

    1. AAcid-fast branching filaments
    2. BGram-positive budding yeasts
    3. CGram-negative budding yeasts
    4. DGram-negative slender bacilli

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22. Question 22 · Hard

    An India ink preparation of CSF shows a few round bodies with clear halos. They vary in size, have no buds and no sharp refractile wall, and the cryptococcal antigen test is negative. The CSF has 300 cells/µL. How should this be reported?

    1. AArtefact, not Cryptococcus
    2. BCandida albicans seen
    3. CCapsulated bacteria seen
    4. DCryptococcus neoformans seen

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23. Question 23 · Medium

    Besides Candida albicans, which yeast can also give a positive germ tube test?

    1. ACandida tropicalis
    2. BCryptococcus neoformans
    3. CCandida dubliniensis
    4. DCandida glabrata

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24. Question 24 · Medium

    Potato dextrose agar is often used for slide cultures of moulds because it:

    1. AInhibits contaminating bacteria
    2. BSpeeds the germ tube test
    3. CConverts moulds to yeasts
    4. DEncourages spores and pigment

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25. Question 25 · Easy

    Routine fungal cultures are usually held for how long before being reported as 'no growth'?

    1. A4 weeks
    2. B7 days
    3. C3 months
    4. D48 hours

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