MRCOG Part 1 · Microbiology · Other Important Concepts

Other Infections: MRCOG Part 1 questions and key facts

Actinomyces israelii is an anaerobic Gram-positive commensal associated with chronic pelvic infection in women with an intrauterine contraceptive device. Group B Streptococcus is the foremost bacterial cause of early-onset neonatal sepsis and meningitis, acquired vertically during labour. Bacterial vaginosis results from an overgrowth of Gardnerella vaginalis and other anaerobes with a loss of normal lactobacilli, producing diagnostic clue cells. Viral questions test genome type (HIV and Zika are single-stranded RNA; herpes simplex, adenovirus and variola are double-stranded DNA), HIV structure and replication (gp120 binding CD4, reverse transcriptase, integrase, p24 antigen), and the timing of seroconversion. Zika is a flavivirus linked to congenital abnormalities, whereas Molluscum contagiosum is a cutaneous poxvirus.

The Uterio bank has 28 questions on this topic. 3 arebelow in full; the syllabus detail is in the Microbiology study guide.

How the exam tests Other Infections

  • Identifying neonatal meningitis and sepsis aetiologies, contrasting early-onset Group B Streptococcus against Escherichia coli.
  • Distinguishing viral genome types, contrasting single-stranded RNA viruses like HIV and Zika with double-stranded DNA viruses like herpes simplex, adenovirus, and variola.
  • Recognising bacterial vaginosis from thin discharge with a fishy odour, absent normal lactobacillary flora, and clue cells on microscopy.
  • Identifying HIV viral markers and therapeutic targets, including p24 capsid antigen, gp120 attachment to CD4, and integrase inhibition by raltegravir.

High-yield facts

  1. Actinomyces israelii is an anaerobic Gram-positive bacterium strongly associated with pelvic infection in women with an intrauterine contraceptive device.
  2. Group B Streptococcus is acquired vertically from the maternal genital tract during labour, causing early-onset neonatal sepsis and meningitis within days of birth.
  3. Escherichia coli represents the second most common bacterial cause of early-onset neonatal meningitis, particularly affecting preterm neonates.
  4. Human immunodeficiency virus is a retrovirus possessing a single-stranded RNA genome converted into double-stranded DNA by viral reverse transcriptase.
  5. The HIV envelope glycoprotein gp120 binds specifically to the CD4 receptor on host T-helper lymphocytes to facilitate viral entry.
  6. Fourth-generation HIV tests detect p24 capsid antigen plus antibody, diagnosing acute infection earlier than antibody-only tests because p24 rises first.
  7. Acute HIV seroconversion illness typically manifests two to four weeks following initial exposure, mimicking infectious mononucleosis.
  8. Raltegravir is an antiretroviral agent that acts as an integrase inhibitor, preventing integration of viral DNA into host chromosomes.
  9. Zika virus is classified genetically as a positive-sense single-stranded RNA flavivirus linked to congenital abnormalities.
  10. Molluscum contagiosum is a poxvirus causing umbilicated pearly papules on the vulva, spread by skin-to-skin contact including sexual contact.
  11. Bacterial vaginosis features falling Lactobacillus species, thin greyish discharge with a fishy odour, clue cells on microscopy, and overgrowth of Gardnerella vaginalis.
  12. Marked peripheral blood lymphocytosis classically indicates an underlying viral infection, whereas acute bacterial infection typically elicits a neutrophilia.

Common pitfalls

  • Confusing p24 antigen with gp120, as p24 forms the internal viral core whereas gp120 mediates external host CD4 receptor binding.
  • Assuming pelvic actinomycosis arises as a conventional sexually transmitted infection rather than opportunistic colonisation around an intrauterine device.
  • Assuming antibodies appear before p24 antigen; in acute infection p24 antigen rises first, which is why fourth-generation tests shorten the window period.
  • Choosing Escherichia coli over Group B Streptococcus for meningitis in a neonate two days old; Group B Streptococcus is the leading cause.

Practice questions: Other Infections

Single best answer, exactly as the exam asks. Commit to an option before opening the explanation.

Question 1

Microbiology

Which enzyme is essential for the replication of HIV within a host cell?

  1. ALigase
  2. BReverse transcriptase
  3. CPrimase
  4. DTopoisomerase
  5. EEndonuclease
Show answer and explanation

Correct answer: B. Reverse transcriptase

Key takeaway

HIV, being a retrovirus, requires the enzyme reverse transcriptase to convert its RNA genome into DNA, enabling integration into the host cell genome and subsequent replication.

Human Immunodeficiency Virus (HIV) is a retrovirus, meaning its genetic material is RNA. For the virus to replicate within a host cell (primarily CD4+ T lymphocytes), its RNA must first be converted into DNA. This crucial step is catalysed by a unique viral enzyme called reverse transcriptase. This enzyme synthesises a complementary DNA (cDNA) strand from the viral RNA template, and then synthesises a second DNA strand, creating double-stranded DNA which can then be integrated into the host cell's genome by another viral enzyme, integrase.

Ligase is an enzyme that joins DNA fragments, essential in general DNA replication and repair but not specific to the retroviral RNA-to-DNA conversion. Primase synthesises RNA primers necessary for initiating DNA synthesis by DNA polymerase. Topoisomerase manages DNA supercoiling during replication and transcription. Endonuclease cuts DNA strands internally; while HIV uses an integrase (a type of endonuclease/recombinase) to insert its DNA into the host genome, reverse transcriptase is the enzyme responsible for creating that DNA from RNA in the first place.

Question 2

Microbiology

Approximately how long after initial HIV infection does the seroconversion illness typically occur?

  1. AWithin 7 days
  2. B2-4 weeks
  3. C2-3 months
  4. D6-12 months
  5. E1-2 years
Show answer and explanation

Correct answer: B. 2-4 weeks

Key takeaway

The symptomatic illness associated with HIV seroconversion typically manifests 2-4 weeks after the initial infection.

The HIV seroconversion illness, also known as acute retroviral syndrome, is a symptomatic illness that occurs in a significant proportion of individuals shortly after acquiring HIV infection. This illness coincides with a high level of viral replication and the initial development of an immune response, including the production of antibodies (seroconversion). Clinical features often mimic glandular fever (infectious mononucleosis), including fever, lethargy, sore throat, rash, and lymphadenopathy. This typically occurs 2-4 weeks after the initial exposure to the virus.

The period within 7 days is generally too early for the adaptive immune response leading to seroconversion symptoms to manifest. While antibody tests might become positive 2-3 months after infection (representing the 'window period' for some tests), the acute symptomatic illness occurs much earlier. The timeframes of 6-12 months and 1-2 years are well beyond the acute seroconversion phase and relate more to the chronic phase of HIV infection, potentially progressing towards AIDS if untreated.

Question 3

Microbiology

During the acute seroconversion phase of HIV infection, which viral marker is typically detectable in the serum before antibodies are produced?

  1. Agp120 antibody
  2. Bp24 antigen
  3. CCCR5 coreceptor
  4. DReverse transcriptase enzyme
  5. EIntegrated proviral DNA
Show answer and explanation

Correct answer: B. p24 antigen

Key takeaway

During the acute seroconversion phase of HIV infection, the viral core protein p24 antigen is detectable in the blood before HIV-specific antibodies are produced, making it an important early diagnostic marker.

The p24 antigen is a structural protein that forms the core or capsid of the HIV virus. High levels of viral replication occur during the acute phase of infection, leading to detectable levels of p24 antigen in the bloodstream. This occurs during the 'window period' after infection but before the body has mounted a detectable antibody response. Modern fourth-generation HIV tests detect both p24 antigen and HIV antibodies, allowing for earlier diagnosis than antibody-only tests.

gp120 antibody is an antibody produced by the host's immune system against the HIV envelope glycoprotein gp120; its appearance signifies seroconversion but occurs later than the detection of p24 antigen. CCR5 coreceptor is a human protein on the surface of immune cells that HIV uses for entry; it is not a viral marker measured for diagnosis. Reverse transcriptase is a viral enzyme essential for replication, and integrated proviral DNA is the viral genetic material incorporated into the host cell genome; neither is routinely measured in serum for diagnosing acute infection.

25 more other infections questions in the bank

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What the syllabus covers under other infections

  • Actinomycosis
  • Group B Streptococcus
  • Other Viruses

Read the full section in the Microbiology guide, or check the next exam dates.