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Microbiology medical question bank

Practise every microbiology topic with explained SBAs.

Recall Engine has 1,000+ microbiology questions across 60+ topics in 5 body systems. Every question explains each option.

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Microbiology by body system

Microbiology has 60+ topics in 5 body systems. Each section shows how many questions it has and every topic it covers.

Respiratory

50+ questions across 3 topics. It also has 3 topic notes.

  • Community-acquired pneumonia
  • Pneumonia: organisms and host defence
  • Viral respiratory infections: influenza

Gastrointestinal and liver

50+ questions across 2 topics. It also has 2 topic notes.

  • Foodborne and waterborne infections
  • GI tract infections: organisms and presentations

Skin

10+ questions across 1 topic. It also has 1 topic note.

  • Leprosy

Blood and cancer

10+ questions across 1 topic. It also has 1 topic note.

  • Oncogenic viruses: mechanisms

Infection and immunity

1,000+ questions across 50+ topics. It also has 50+ topic notes.

  • Applied stool analysis
  • Arboviruses and viral haemorrhagic fevers
  • Aspergillus, Cryptococcus, and Pneumocystis
  • Bacteria: virulence factors and toxins
  • Bacterial infections: diagnosis
  • Bone, joint, and soft-tissue infections
  • Brucellosis
  • Cryptosporidium: life cycle, diagnosis, and immune status
  • Cytomegalovirus and Epstein-Barr virus
  • Emerging infections: drivers and examples
  • Enterobius: sampling, reinfection, and household care
  • Fasciola, Diphyllobothrium, and Hymenolepis
  • Filariasis
  • Fungi: specimens, microscopy, and culture
  • Giardia and Entamoeba: stages and diagnosis
  • Gram-positive infections
  • Healthcare-associated infections
  • Helminth infections
  • Helminths and protozoa: classification and microscopy
  • Hepatitis C virus: virology and testing
  • Herpes simplex virus
  • Herpetic whitlow: diagnosis and management
  • Histological and microbiological stains: targets and colours
  • HIV: virion, replication, and testing
  • Immunocompromised host infections
  • Infectious agents: classes and transmission
  • Infectious gastroenteritis: viral and bacterial causes
  • Intestinal nematodes: life cycles and diagnosis
  • Introduction to clinical microbiology
  • Introduction to infectious agents
  • Introduction to medical parasitology and malaria
  • Leishmania amastigote and Taenia morphology
  • Leishmania: life cycle and diagnosis
  • Malaria
  • Meningitis: CSF tests and meningococcal disease
  • Meningitis: organisms and host factors
  • Mycobacterium tuberculosis: cell wall and spread
  • Parasite identification
  • Parasites: groups and life-cycle terms
  • Plasmodium: life cycle and diagnosis
  • Pneumococcus: virulence and host defence
  • RSV bronchiolitis: diagnosis
  • Schistosoma species infection
  • Schistosoma: morphology and transmission
  • Streptococcus pyogenes: infections and diagnosis
  • Taenia: morphology and transmission
  • Taeniasis: diagnosis and treatment
  • The innate and adaptive interface
  • Tuberculosis
  • Tuberculosis: granuloma and diagnosis
  • Vaccines and passive immunisation
  • Varicella zoster virus
  • Viral infections: diagnosis
  • Viral transmission: routes and prevention
  • Viruses and cancer
  • Viruses: entry, spread, and persistence
  • Viruses: structure, replication, and classification

Microbiology sample questions

These 5 are published microbiology questions shown exactly as students see them. Open an answer to see why it is right and why every other option is wrong.

Question 1 · Microbiology, Preclinical

A 42-year-old farmer who has irrigated fields by hand since childhood presents with haematemesis from bleeding oesophageal varices. He has massive splenomegaly but no jaundice or ascites. Albumin is 39 g/L, bilirubin and INR are normal, and hepatitis B and C tests are negative. Ultrasound shows a normal-sized smooth liver with thick echogenic bands around the portal vein branches. Which mechanism best explains his portal hypertension?

  1. A. Adult worms lodged in and blocking the hepatic veins
  2. B. Prehepatic block from portal vein thrombosis alone
  3. C. Presinusoidal block from egg-induced periportal fibrosis
  4. D. Sinusoidal fibrosis with regenerative nodules from cirrhosis
  5. E. Post-sinusoidal block from hepatic vein thrombosis
Show the answer and explanation

Answer: C. Presinusoidal block from egg-induced periportal fibrosis

Lifelong freshwater exposure, varices, and massive splenomegaly with preserved liver function and thick periportal bands are typical of chronic intestinal schistosomiasis (S. mansoni). Eggs carried in the portal blood lodge in presinusoidal portal venules and provoke granulomas that heal into periportal (pipe-stem) fibrosis. The block is presinusoidal, so hepatocytes and liver function are spared.

Why the other options are wrong

  • A. Adult worms lodged in and blocking the hepatic veins: Adult schistosomes live in mesenteric venules and do little harm; the damage comes from eggs trapped in portal tracts.
  • B. Prehepatic block from portal vein thrombosis alone: Isolated portal vein thrombosis lacks the periportal fibrotic bands seen here; in this farmer the bands point to egg-induced fibrosis.
  • D. Sinusoidal fibrosis with regenerative nodules from cirrhosis: Cirrhosis distorts the whole liver with nodules and impairs synthesis; his liver is smooth with normal albumin, bilirubin, and INR.
  • E. Post-sinusoidal block from hepatic vein thrombosis: Hepatic vein thrombosis causes painful hepatomegaly and ascites; he has no ascites and the fibrosis follows the portal tracts.

Key point: Chronic S. mansoni infection causes egg granulomas and periportal fibrosis with presinusoidal portal hypertension, splenomegaly, and varices, while liver synthetic function is usually preserved.

References

Question 2 · Microbiology, Preclinical

Paired sera from a 35-year-old man with a febrile illness are titrated together by latex agglutination with antigen-coated particles. The acute serum clumps at 1/2 and 1/4 and stays smooth from 1/8 to 1/64. The serum taken 14 days later clumps from 1/2 to 1/32 and is smooth at 1/64. Controls read correctly. Which interpretation fits?

  1. A. A sixty-fourfold rise in titre, from 1/2 to 1/64
  2. B. Eightfold rise, supporting recent infection
  3. C. A fourfold fall in titre, pointing to past infection
  4. D. A twofold rise in titre, within assay variation
  5. E. No change in titre, as both sera agglutinate
Show the answer and explanation

Answer: B. Eightfold rise, supporting recent infection

The titre is the highest dilution of serum that still gives visible agglutination of the antigen-coated latex. The acute titre is 1/4 and the convalescent titre is 1/32, an eightfold (three-dilution) rise. A fourfold or greater rise in paired sera taken about 2 weeks or more apart, tested on the same run, supports recent infection.

Why the other options are wrong

  • A. A sixty-fourfold rise in titre, from 1/2 to 1/64: This misreads both endpoints: the acute titre is 1/4, and 1/64 did not agglutinate, so the convalescent titre is 1/32.
  • C. A fourfold fall in titre, pointing to past infection: The titre went up, not down: the convalescent serum agglutinates at higher dilutions than the acute serum.
  • D. A twofold rise in titre, within assay variation: Counting doubling steps from 1/4 to 1/32 gives three steps, an eightfold rise, not a single step.
  • E. No change in titre, as both sera agglutinate: Both sera clump at low dilutions, but the titre is the highest dilution that clumps, and that moved from 1/4 to 1/32.

Key point: An antibody titre is the highest serum dilution still giving a visible reaction; a fourfold or greater rise between acute and convalescent sera tested together supports recent infection.

References

Question 3 · Microbiology, Preclinical

A 31-year-old man has a hot, fluctuant 4 cm abscess on his thigh that is about to be incised and drained. He has no fever and no other site of infection. The surgeon wants the culture to show the organism inside the abscess and not the bacteria living on the surrounding skin. Which specimen best meets that aim?

  1. A. A swab of the drained cavity taken at next-day review
  2. B. A nasal swab for staphylococcal carriage before surgery
  3. C. A set of blood cultures taken from the opposite arm
  4. D. Pus aspirated into a sterile syringe before incision
  5. E. A dry swab rubbed over the skin covering the abscess
Show the answer and explanation

Answer: D. Pus aspirated into a sterile syringe before incision

The aim is to grow the organism from the site of infection. Pus aspirated into a sterile syringe comes from inside the abscess, is collected before the cavity is opened to skin flora, and arrives in a sterile container. That is the definition of a good specimen: the right site, no contamination, sterile handling, and prompt transport (refrigerated if delayed).

Why the other options are wrong

  • A. A swab of the drained cavity taken at next-day review: By the next day the open cavity is colonised by skin and environmental organisms, and swabs are a poorer specimen than pus taken at the start.
  • B. A nasal swab for staphylococcal carriage before surgery: A nasal swab shows what the patient carries in the nose, which may differ from what caused this thigh abscess.
  • C. A set of blood cultures taken from the opposite arm: Blood cultures detect organisms in the bloodstream; an afebrile man with a localised abscess is unlikely to be bacteraemic, so the yield would be low.
  • E. A dry swab rubbed over the skin covering the abscess: A surface swab samples the skin flora sitting on top of the lesion; the organism inside the closed abscess is reached only by sampling the pus itself.

Key point: A good culture specimen comes from the infected site itself, ideally fluid or tissue aspirated into a sterile container before antibiotics, rather than a surface swab that mostly collects neighbouring flora.

References

  • Culture, MSD Manual Professional Edition

Question 4 · Microbiology, Preclinical

A 27-year-old woman attends her antenatal booking visit at 10 weeks of pregnancy. She feels well and has had no rash or contact with a rash illness. Her serum shows rubella IgG positive and rubella IgM negative. Which interpretation fits these results?

  1. A. Susceptible to rubella and needing protection
  2. B. Acute rubella acquired in early pregnancy
  3. C. Ongoing persistent infection marked by the IgG
  4. D. Fetal infection shown by the maternal IgG
  5. E. Immune from past vaccination or infection
Show the answer and explanation

Answer: E. Immune from past vaccination or infection

Rubella IgG without IgM, in a well woman with no recent exposure, marks immunity from vaccination or past wild infection. This is one of the main uses of IgG serology: determining susceptibility. For persisting viruses (herpesviruses and HIV) the same finding means ongoing infection, so the meaning of IgG depends on the virus.

Why the other options are wrong

  • A. Susceptible to rubella and needing protection: Susceptibility is shown by absent rubella IgG; her IgG is positive, so she already has protective antibody.
  • B. Acute rubella acquired in early pregnancy: Acute rubella would usually give IgM, and she has no IgM, no rash, and no contact, so recent infection is not supported.
  • C. Ongoing persistent infection marked by the IgG: IgG marks persistent infection only for viruses that stay in the body, such as CMV, VZV, and HIV; rubella IgG in a healthy adult marks immunity.
  • D. Fetal infection shown by the maternal IgG: Maternal IgG shows the mother's immune state, not fetal infection; fetal infection is shown by the infant's own IgM after birth.

Key point: IgG without IgM to a virus that is cleared after infection indicates immunity; for persisting viruses such as CMV, VZV, and HIV, IgG instead marks ongoing infection.

References

Question 5 · Microbiology, Preclinical

An 18-year-old university student develops fever, severe headache, neck stiffness, and a non-blanching rash 2 days after a sore throat. He has no ear, sinus, or dental infection, no head injury, and no previous surgery. Blood cultures and CSF grow gram-negative diplococci. Which route most likely carried the organism to his meninges?

  1. A. Contiguous spread from an infected sinus or mastoid
  2. B. Retrograde axonal transport along peripheral nerves
  3. C. Direct inoculation through a skull defect or CSF leak
  4. D. Faecal-oral infection followed by viraemia
  5. E. Haematogenous spread after nasopharyngeal invasion
Show the answer and explanation

Answer: E. Haematogenous spread after nasopharyngeal invasion

Gram-negative diplococci in blood and CSF with a non-blanching rash indicate meningococcal disease. Bacteria usually colonise the nasopharynx first; most commonly they then invade the mucosa and reach the meninges through the bloodstream (haematogenous spread), which also explains the positive blood culture. The other routes need a local cause: contiguous spread from sinus, middle ear, or mastoid; direct entry through a wound, neurosurgery, or a skull defect with CSF leak; or, for some viruses such as rabies and herpes simplex, travel along nerves.

Why the other options are wrong

  • A. Contiguous spread from an infected sinus or mastoid: Contiguous spread needs an infected structure next to the meninges, such as a sinus, middle ear, or mastoid; he has none.
  • B. Retrograde axonal transport along peripheral nerves: Travel inside nerve axons is how rabies virus and herpes simplex virus reach the brain, not how meningococci reach the meninges.
  • C. Direct inoculation through a skull defect or CSF leak: Direct entry needs a penetrating wound, neurosurgery, a shunt, or a skull defect with CSF leak, and he has none of these.
  • D. Faecal-oral infection followed by viraemia: Faecal-oral spread with viraemia describes enteroviruses, which cause aseptic meningitis without gram-negative diplococci.

Key point: Most bacterial meningitis follows nasopharyngeal colonisation and haematogenous spread; contiguous spread, direct inoculation, and neural travel each need a specific local route or organism.

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