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Microbiology and Immunology

Microbial diagnostics Practice Questions

20 free Microbial diagnostics practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.

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Question 1 of 20 Medium

A patient with suspected tuberculosis undergoes initial testing. Which diagnostic method is most sensitive for detecting active TB infection?

  1. A Chest radiography
  2. B Sputum acid-fast smear microscopy
  3. C Interferon-gamma release assay
  4. D Sputum nucleic acid amplification

Correct answer: Sputum nucleic acid amplification

NAAT detects M. tuberculosis DNA and provides rapid and sensitive results. It is more reliable than smear alone for active disease.

Question 2 of 20 Medium

A throat swab from a patient with suspected streptococcal pharyngitis undergoes rapid antigen detection testing. What is a key limitation of this test?

  1. A High rate of false negatives
  2. B High rate of false positives
  3. C Cannot detect Group A Streptococcus
  4. D Requires 48 hours for results

Correct answer: High rate of false negatives

Rapid antigen tests for Group A Streptococcus have reduced sensitivity. Negative results often require confirmation with culture.

Question 3 of 20 Medium

A clinician suspects Legionella pneumonia. Which diagnostic test is most specific and rapid?

  1. A PCR of blood
  2. B Culture on blood agar
  3. C Urinary antigen test
  4. D Sputum Gram stain

Correct answer: Urinary antigen test

The urinary antigen test quickly detects L. pneumophila serogroup 1. It is widely used due to high specificity and rapid turnaround.

Question 4 of 20 Medium

A CSF sample from a meningitis patient shows many PMNs but no organisms on Gram stain. Which test is most helpful for rapid diagnosis?

  1. A Culture on chocolate agar
  2. B Latex agglutination test
  3. C PCR for bacterial pathogens
  4. D India ink stain preparation

Correct answer: PCR for bacterial pathogens

PCR can detect bacterial DNA even when Gram stain is negative. It is rapid and highly sensitive for meningitis pathogens.

Question 5 of 20 Medium

A microbiology lab identifies Gram-negative diplococci in a urethral smear from a symptomatic male. What is the next best confirmatory step?

  1. A PCR for Neisseria gonorrhoeae
  2. B Tube coagulase test
  3. C Catalase enzyme test
  4. D Rapid urease test

Correct answer: PCR for Neisseria gonorrhoeae

PCR is sensitive and specific for N. gonorrhoeae and is widely used for confirmation. Culture is less sensitive but still possible.

Question 6 of 20 Medium

A patient presents with fever, rash, and a history of tick exposure. What test is most appropriate for early detection of rickettsial infection?

  1. A Serology for IgG antibodies
  2. B Weil-Felix test
  3. C PCR of blood
  4. D Blood culture

Correct answer: PCR of blood

PCR detects rickettsial DNA early in infection. Serology becomes positive only later in disease.

Question 7 of 20 Medium

A fungal infection is suspected in a patient with chronic cough and cavitary lung lesions. What diagnostic test best identifies fungal cell wall components?

  1. A Galactomannan assay
  2. B Bacterial Gram stain
  3. C Catalase enzyme test
  4. D Tzanck smear cytology

Correct answer: Galactomannan assay

Galactomannan assay detects Aspergillus cell wall components. It aids in diagnosing invasive aspergillosis.

Question 8 of 20 Medium

A patient has profuse watery diarrhea after antibiotic use. Stool ELISA for toxins A and B is negative. Which next test is most appropriate?

  1. A Stool culture for C. difficile
  2. B CT scan of abdomen and pelvis
  3. C NAAT for C. difficile toxin genes
  4. D Serum toxin PCR assay

Correct answer: NAAT for C. difficile toxin genes

NAAT detects toxin genes with high sensitivity. It is often used when ELISA results are negative but suspicion remains high.

Question 9 of 20 Medium

A patient with a bull’s-eye rash is suspected of having Lyme disease. What is the most appropriate initial diagnostic test?

  1. A PCR of blood sample
  2. B ELISA for Borrelia antibodies
  3. C Western blot assay
  4. D Blood culture on agar

Correct answer: ELISA for Borrelia antibodies

ELISA is the first-line serologic screening test. Positive ELISA must be confirmed with Western blot.

Question 10 of 20 Medium

A bronchoalveolar lavage from a patient with suspected Pneumocystis jirovecii pneumonia is analyzed. Which stain best identifies the organism?

  1. A Wright-Giemsa stain
  2. B Gomori methenamine silver stain
  3. C Ziehl-Neelsen acid-fast stain
  4. D Gram stain of smear

Correct answer: Gomori methenamine silver stain

Gomori methenamine silver stain highlights cyst walls of P. jirovecii. It is highly sensitive for diagnosis.

Question 11 of 20 Medium

A blood sample from a patient with suspected malaria is collected. What is the gold standard diagnostic test?

  1. A Indirect ELISA
  2. B Thick and thin blood smears
  3. C PCR for Plasmodium species
  4. D Rapid antigen detection test

Correct answer: Thick and thin blood smears

Thick and thin smears allow visualization and species identification. They remain the gold standard despite rapid test availability.

Question 12 of 20 Medium

A patient with suspected cryptococcal meningitis undergoes lumbar puncture. Which diagnostic test is most sensitive?

  1. A India ink stain of CSF
  2. B Cryptococcal antigen assay
  3. C CSF culture on Sabouraud agar
  4. D Gram stain of CSF

Correct answer: Cryptococcal antigen assay

Cryptococcal antigen testing is highly sensitive, even with low organism burden. India ink stain may miss cases with low fungal counts.

Question 13 of 20 Medium

A stool sample from a child with bloody diarrhea is tested for Shiga toxin. Which method is most appropriate?

  1. A Latex agglutination test
  2. B Stool culture on MacConkey agar
  3. C PCR for Shiga toxin genes
  4. D Catalase test

Correct answer: PCR for Shiga toxin genes

PCR rapidly detects Shiga toxin genes and is highly sensitive. Culture alone may not identify toxin-producing strains.

Question 14 of 20 Medium

A nasopharyngeal swab is obtained from a patient with influenza-like illness. Which test provides the fastest and most accurate diagnosis?

  1. A Rapid antigen test
  2. B PCR-based respiratory panel
  3. C Viral culture
  4. D Hemagglutination inhibition test

Correct answer: PCR-based respiratory panel

PCR-based respiratory panels detect influenza and other respiratory viruses with high sensitivity and rapid turnaround.

Question 15 of 20 Medium

A patient presents with urethritis likely caused by Chlamydia trachomatis. Which test is the diagnostic method of choice?

  1. A Serology for IgM antibodies
  2. B NAAT of urine or swab
  3. C Gram stain of urethral discharge
  4. D Culture on blood agar

Correct answer: NAAT of urine or swab

NAAT is highly sensitive for C. trachomatis and is the preferred diagnostic test. Chlamydia does not appear on Gram stain.

Question 16 of 20 Medium

A patient with chronic cough is suspected of having histoplasmosis. What diagnostic finding supports the diagnosis?

  1. A Broad-based budding yeasts
  2. B Acid-fast bacilli in tissue
  3. C Oval yeasts within macrophages
  4. D Septate branching hyphae

Correct answer: Oval yeasts within macrophages

Histoplasma capsulatum appears as intracellular yeasts within macrophages. This is a key microscopic finding.

Question 17 of 20 Medium

A patient with suspected diphtheria undergoes laboratory evaluation. Which test confirms toxigenicity?

  1. A PCR for Corynebacterium diphtheriae
  2. B Culture on tellurite agar
  3. C Elek test
  4. D Catalase test

Correct answer: Elek test

The Elek test detects diphtheria toxin production. Culture alone cannot determine toxigenicity.

Question 18 of 20 Medium

A patient presents with watery diarrhea after travel to Mexico. Initial stool ova-and-parasite microscopy is negative, but giardiasis remains strongly suspected. Which test most sensitively confirms Giardia lamblia infection?

  1. A PCR stool panel
  2. B Serologic IgG testing
  3. C Blood culture
  4. D Western blot

Correct answer: PCR stool panel

A stool PCR (multiplex GI pathogen panel) is the most sensitive method for confirming Giardia and can detect infection even when microscopy is negative, because cyst and trophozoite shedding is intermittent. Serologic IgG cannot distinguish current from past infection, and Giardia is a luminal intestinal parasite that is not recovered by blood culture or identified by Western blot.

Question 19 of 20 Medium

A biopsy from a patient with suspected herpesvirus infection shows multinucleated giant cells. What diagnostic test corresponds to this finding?

  1. A Tzanck smear
  2. B Gomori silver stain
  3. C India ink preparation
  4. D Cold agglutinin test

Correct answer: Tzanck smear

Tzanck smear reveals multinucleated giant cells in herpes infections. It is rapid but not specific for viral type.

Question 20 of 20 Medium

A 22-year-old patient presents with a genital ulcer suspicious for syphilis. Which is the most appropriate initial serologic screening test?

  1. A FTA-ABS
  2. B Dark-field microscopy of lesion exudate
  3. C VDRL or RPR
  4. D PCR of blood

Correct answer: VDRL or RPR

Nontreponemal serologic tests (VDRL or RPR) are the standard initial screening tests for syphilis; a reactive result is then confirmed with a treponemal test such as FTA-ABS. Dark-field microscopy and PCR are direct-detection methods rather than serologic screens, and FTA-ABS is a confirmatory treponemal test, not an initial screen.

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