Virology: specimens and processing

21 Kerala PSC Lab Technician Gr II practice questions on Virology: specimens and processing, part of Microbiology. The first 21 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 · Easy

   Negri bodies are looked for in which part of the brain?

   1. AChoroid plexus
   2. BPituitary gland
   3. CHippocampus and cerebellum
   4. DFrontal white matter

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   Answer: C · Hippocampus and cerebellum

   Negri bodies are eosinophilic cytoplasmic inclusions of rabies virus, found in the pyramidal neurons of the hippocampus (Ammon's horn) and in the Purkinje cells of the cerebellum. Smears or sections from these areas are stained by Seller's stain or examined by direct fluorescent antibody, which is more sensitive. They are absent in about a fifth of proven cases, so a negative smear does not exclude rabies. The brain is handled with full precautions.

   • A: The choroid plexus is not sampled for Negri bodies.
   • B: The pituitary is unrelated to rabies diagnosis.
   • D: The frontal white matter is not where the inclusions concentrate.

2. Question 2 · Hard

   A man bitten by a stray dog 2 months ago is admitted with hydrophobia. Which approach best confirms rabies during life?

   1. ACulture of a throat swab in Vero cells
   2. BRT-PCR on saliva and a nuchal skin biopsy
   3. CNegri bodies in a stained peripheral blood smear
   4. DRabies antibody in a single serum sample

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   Answer: B · RT-PCR on saliva and a nuchal skin biopsy

   Antemortem confirmation rests on detecting the virus itself: RT-PCR on saliva, and direct fluorescent antibody or PCR on a nuchal skin biopsy that includes hair follicles, where nerve endings carry the antigen. Several saliva samples are taken, as shedding is intermittent, and a corneal impression smear may be added. Serum antibody appears late and is unhelpful in an unvaccinated patient already ill, and antibody after vaccination cannot be separated from infection without CSF testing. The virus does not circulate in blood, so blood smears show nothing.

   • A: Culture is slow, hazardous and not used for diagnosis during life.
   • C: There is no viraemia, so blood smears are of no value.
   • D: Antibody appears late and a single titre cannot confirm the illness.

3. Question 3 · Medium

   Why is repeated freezing and thawing of a specimen for virus isolation avoided?

   1. AIt makes the container crack
   2. BIt washes out the antibiotics
   3. CIt raises the risk to the worker
   4. DIt lowers the infectivity of the virus

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   Answer: D · It lowers the infectivity of the virus

   Each freeze-thaw cycle forms ice crystals that damage viral envelopes and nucleic acid, so the titre falls and isolation may fail. The specimen is therefore divided into small aliquots when it arrives, and one aliquot is thawed for each test. Aliquots also leave material for a repeat test or for referral to a reference laboratory. Thawing is done quickly, at room temperature or in a water bath, and the sample is then kept cold.

   • A: Containers seldom crack, and that is not the reason for the rule.
   • B: Freezing does not remove the antibiotics from the medium.
   • C: The hazard to the worker is the same whether the sample is thawed once or many times.

4. Question 4 · Easy

   Viral transport medium contains bovine serum albumin or gelatin in order to:

   1. ABuffer the pH
   2. BSuppress contaminating bacteria
   3. CStabilise the virus
   4. DLyse host cells

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   Answer: C · Stabilise the virus

   A protein such as albumin or gelatin protects the virus against inactivation during transport and against the shock of freezing and thawing. The other ingredients have their own roles: a buffered salt solution holds the pH, and antibiotics with an antifungal agent hold back bacteria and fungi so that cell cultures are not spoilt. Host cells in the specimen are wanted intact for immunofluorescence, so nothing in the medium lyses them. The swab tip is broken off into the medium.

   • A: The buffered salt solution holds the pH.
   • B: Antibiotics in the medium suppress bacteria.
   • D: Infected cells are kept intact for antigen detection.

5. Question 5 · Medium

   In a direct fluorescent antibody test on a nasopharyngeal aspirate, what is detected?

   1. AAntibody in the patient's serum
   2. BViral antigen in infected cells
   3. CViral nucleic acid in the cells
   4. DLive virus growing in culture

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   Answer: B · Viral antigen in infected cells

   The smear of exfoliated respiratory cells is fixed and flooded with a fluorescein-labelled antibody against the virus, and apple-green fluorescence in the cytoplasm or nucleus shows that viral antigen is present. The report is ready within hours, long before culture, which is why it was widely used for respiratory syncytial virus and influenza. The specimen must contain enough epithelial cells, so an aspirate or a properly rotated swab is needed. The indirect method adds a labelled anti-human antibody and is more sensitive.

   • A: Serum antibody is measured by ELISA or other serological tests.
   • C: Nucleic acid is detected by PCR, not by an antibody stain.
   • D: Culture is a separate, slower method.

6. Question 6 · Easy

   A swab in viral transport medium cannot be processed within 48 hours. At what temperature should it be held?

   1. A4°C
   2. BRoom temperature
   3. CMinus 20°C
   4. DMinus 70°C

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7. Question 7 · Medium

   Why is fluid for virus isolation taken from a fresh vesicle rather than from a crusted lesion?

   1. ACrusted lesions are sterile and acellular
   2. BFresh vesicle fluid contains more antibody
   3. CFresh vesicles hold the most live virus
   4. DCrusts are too dry for transport medium

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8. Question 8 · Easy

   Guarnieri bodies are inclusion bodies seen in cells infected with:

   1. AHerpes simplex virus
   2. BMeasles virus
   3. CPoxviruses such as vaccinia
   4. DRabies virus

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9. Question 9 · Hard

   A 10-day-old baby has jaundice, petechiae, hepatosplenomegaly and periventricular calcification on a cranial scan. A stained urine sediment shows greatly enlarged cells with large intranuclear inclusions surrounded by a clear halo. Which virus is the most likely cause?

   1. ARubella virus
   2. BCytomegalovirus
   3. CVaricella-zoster virus
   4. DHerpes simplex virus

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10. Question 10 · Easy

    For virus isolation and PCR, which swab is preferred?

    1. ACharcoal-coated cotton swab
    2. BCalcium alginate on a wire shaft
    3. CCotton on a wooden shaft
    4. DFlocked nylon on a plastic shaft

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11. Question 11 · Medium

    In what are brain impression smears fixed before a direct fluorescent antibody test for rabies?

    1. ABouin's fluid
    2. B10% formal saline
    3. CFlame heat
    4. DCold acetone

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12. Question 12 · Hard

    A nasopharyngeal swab in viral transport medium was left at room temperature over a weekend before reaching the laboratory. Influenza RT-PCR is positive, but culture in MDCK cells, which support influenza, shows no virus. The best explanation is that the:

    1. ASwab was taken from the wrong site
    2. BVirus lost infectivity, but its RNA survived
    3. CAntibiotics in the transport medium killed it
    4. DRT-PCR result is a contamination artefact

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13. Question 13 · Easy

    In acute flaccid paralysis surveillance for polio, which specimens are collected?

    1. AOne throat swab and one urine
    2. BOne CSF sample at admission
    3. CPaired sera, two weeks apart
    4. DTwo stool samples, 24 hours apart

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14. Question 14 · Hard

    After the death of a patient with suspected rabies, a piece of brain is sent in 10% formal saline for virus isolation. What is wrong, and what should have been used?

    1. AFormalin lyses Negri bodies; send it in phenol saline
    2. BFormalin kills the virus; send it in 50% glycerol saline
    3. CSaline dries the tissue; send it in absolute alcohol
    4. DNothing is wrong; formal saline suits isolation

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15. Question 15 · Hard

    A direct fluorescent antibody test for rabies is run on brain impression smears. The negative control smear from a normal brain shows bright apple-green granules scattered over the whole field. The patient's smear also shows fluorescence. The correct action is:

    1. AReport positive after examining twice as many fields
    2. BReport the patient's smear as positive for rabies antigen
    3. CReport negative, since the control shows the stain fails
    4. DReject the run and repeat it with proper washing and a fresh conjugate

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16. Question 16 · Medium

    Throat swab and CSF from a suspected Nipah patient are sent to the reference laboratory. They must be packed in:

    1. AA triple packaging system
    2. BA screw-capped tube in an envelope
    3. CA tube placed in 10% formalin
    4. DA single sealed plastic bag

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17. Question 17 · Medium

    In an indirect immunofluorescence test, which reagent carries the fluorescein label?

    1. AAnti-human immunoglobulin
    2. BMounting fluid
    3. CPatient's serum antibody
    4. DViral antigen on the slide

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18. Question 18 · Medium

    A child has small pearly papules with a central dimple; material squeezed from one shows large cytoplasmic inclusions. What is the diagnosis?

    1. ACommon warts
    2. BHerpes zoster
    3. CChickenpox
    4. DMolluscum contagiosum

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19. Question 19 · Medium

    For a diagnosis by rising antibody titre, the acute and convalescent serum samples should be taken:

    1. AOn the same day, from two different veins
    2. BTwo days apart, during the fever
    3. COnly after the patient has fully recovered
    4. DAs early as possible, then 2-3 weeks later

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20. Question 20 · Easy

    Which virus produces Cowdry type A intranuclear inclusion bodies?

    1. AHerpes simplex virus
    2. BVaccinia virus
    3. CRabies virus
    4. DMolluscum contagiosum virus

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21. Question 21 · Easy

    A specimen for virus isolation that will reach the laboratory within 24 hours should be kept at:

    1. AMinus 20°C
    2. BRoom temperature
    3. C37°C
    4. D2-8°C

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