MLT GlobeVirology: HIV & retroviruses
20 Virology MCQs on HIV & retroviruses with answers and explanations.
Which virus causes acquired immunodeficiency syndrome (AIDS) worldwide?
HIV-1 causes most AIDS worldwide (HIV-2 mainly in West Africa). The old name HTLV-III is obsolete; HTLV-1 causes adult T-cell leukaemia.
Which single exposure carries the highest risk of HIV transmission?
Transfusion of infected blood transmits HIV in over 90% of cases, which is why donations are screened. Needlestick risk is about 0.3%; casual household or workplace contact carries no risk.
Which retroviral enzyme, encoded by the pol gene, inserts the viral DNA copy into the host chromosome?
Integrase cuts host DNA and joins the viral cDNA to form the provirus; drugs such as dolutegravir inhibit it. Reverse transcriptase makes the DNA copy first.
A retrovirus must copy its RNA genome into DNA before integration. Which enzyme performs this step?
Reverse transcriptase is an RNA-dependent DNA polymerase that makes DNA from RNA. Integrase then inserts the DNA into the host genome but does not copy it.
To follow an HIV patient's response to antiretroviral therapy, the amount of virus in plasma is best measured by:
Viral load is quantified as HIV RNA copies/mL by real-time RT-PCR. Antibody tests only show infection, and sequencing is used for resistance testing.
HIV mainly infects and destroys which cells?
HIV binds CD4 with a chemokine co-receptor; loss of CD4 T cells causes immunodeficiency.
In an adult with HIV infection, a CD4 count below which level is used to define AIDS?
A CD4 count below 200 cells/µL (or an AIDS-defining illness) meets the definition of AIDS.
Which statement about HIV is NOT true?
HIV is a lentivirus, not an oncoretrovirus like HTLV-1; the cancers in AIDS result from immunosuppression and other viruses. It carries reverse transcriptase and spreads mainly sexually.
A newly diagnosed HIV-positive man asks how quickly his disease may progress. Which baseline test best predicts the rate of progression?
The set-point plasma HIV RNA level strongly predicts the speed of CD4 decline and progression to AIDS; together with the CD4 count it guides management. Antibody tests only confirm infection.
A combined antiretroviral regimen contains zidovudine, lamivudine and a boosted protease inhibitor. Which two viral enzymes does it target?
Zidovudine and lamivudine are nucleoside reverse transcriptase inhibitors; the protease inhibitor blocks maturation cleavage. gp120/gp41 are envelope targets of entry inhibitors.
Which HIV entry inhibitor works by blocking the host CCR5 co-receptor?
Maraviroc is a CCR5 antagonist, effective only against R5-tropic HIV, so tropism testing is needed first. Enfuvirtide targets gp41.
A blood film from an adult with hypercalcaemia shows lymphocytes with cloverleaf ("flower") nuclei. Which virus causes this leukaemia?
HTLV-1 causes adult T-cell leukaemia/lymphoma, with characteristic flower cells and hypercalcaemia; it is endemic in Japan, the Caribbean and parts of Africa.
Rous sarcoma virus, which carries the src oncogene, is best classified as which kind of virus?
RSV is an avian retrovirus; its v-src gene was the first oncogene discovered. It is enveloped and has +ssRNA copied into DNA by reverse transcriptase.
A fourth-generation HIV screening immunoassay detects:
Adding p24 antigen detection shortens the window period compared with antibody-only tests, because p24 appears before antibodies. Reactive results go on to a differentiation assay and, if needed, RNA testing.
A fourth-generation HIV-1/2 antigen/antibody immunoassay is reactive. What is the next step in the recommended algorithm?
The current CDC algorithm follows a reactive screen with an HIV-1/HIV-2 differentiation assay. Western blot is no longer used as the confirmatory test.
How is HIV infection diagnosed in a 2-month-old infant born to an HIV-positive mother?
Maternal IgG antibodies cross the placenta and may last up to 18 months, so antibody tests cannot diagnose infant infection. Virologic tests (NAT) are required.
The HIV virion contains which genetic material?
Retroviruses are diploid, carrying two identical positive-sense RNA strands that are copied into DNA by reverse transcriptase.
A 4th-generation HIV screen is reactive, but the HIV-1/HIV-2 differentiation assay is negative. What should be done next?
This pattern may be acute HIV infection, when p24 antigen is present before antibodies. HIV-1 NAT resolves it: a positive result shows acute HIV-1 infection.
Why is it important to tell HIV-2 from HIV-1 infection?
HIV-2 is naturally resistant to non-nucleoside reverse transcriptase inhibitors, and many HIV-1 RNA assays do not quantify it. Treatment and monitoring must therefore differ.
HIV genotypic drug-resistance testing usually fails when the plasma HIV-1 RNA is below:
Sequencing needs enough viral RNA to amplify the pol gene, so most assays require a viral load of roughly 500–1000 copies/mL or more. A load of 20 copies/mL is too low to sequence.