Which test is recommended for Chlamydia trachomatis on a vaginal swab?

Nucleic acid amplification test

Culture on modified Thayer-Martin agar

Gram stain for intracellular organisms

Genital tract

10 ASCP MLS practice questions on Genital tract, part of Microbiology. The first 10 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Easy
Which test is recommended for Chlamydia trachomatis on a vaginal swab?
1. AWet mount examination
2. BCulture on modified Thayer-Martin agar
3. CNucleic acid amplification test
4. DGram stain for intracellular organisms
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Answer: C · Nucleic acid amplification test
Nucleic acid amplification tests (NAATs) are the most sensitive and specific tests for C. trachomatis and are recommended by CDC for screening and diagnosis. Self- or clinician-collected vaginal swabs are the preferred specimen in women, and first-catch urine in men. Most NAATs also test for N. gonorrhoeae in the same specimen. C. trachomatis is an obligate intracellular bacterium, so it cannot grow on artificial media and culture needs cell lines such as McCoy cells.
- A: A wet mount detects trichomonads, yeasts and clue cells, not Chlamydia.
- B: Thayer-Martin agar is for N. gonorrhoeae; Chlamydia needs living cells to grow.
- D: Chlamydiae stain poorly with the Gram stain and are too small to see reliably.
Question 2 · Medium
A Gram-stained vaginal smear shows few lactobacilli and many small gram-variable rods and curved gram-variable rods. A Nugent score of 8 indicates:
1. ABacterial vaginosis
2. BNormal vaginal flora
3. CIntermediate flora
4. DCandida vaginitis
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Answer: A · Bacterial vaginosis
The Nugent score grades a vaginal Gram stain for three morphotypes: large gram-positive rods (lactobacilli), small gram-variable rods (Gardnerella and Bacteroides types) and curved gram-variable rods (Mobiluncus). Fewer lactobacilli and more of the other two raise the score. A score of 0 to 3 is normal, 4 to 6 is intermediate, and 7 to 10 is bacterial vaginosis. The Nugent score is the laboratory reference method for diagnosing BV.
- B: Normal flora scores 0 to 3, with many lactobacilli.
- C: Intermediate flora scores 4 to 6.
- D: Candida is seen as yeasts and pseudohyphae and is not part of the Nugent score.
Question 3 · Easy
In modified Thayer-Martin agar, which group of organisms does vancomycin inhibit?
1. AYeasts and molds
2. BGram-positive bacteria
3. CGram-negative bacilli
4. DProteus species
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Answer: B · Gram-positive bacteria
Modified Thayer-Martin agar is chocolate agar with antimicrobials that suppress normal flora while letting pathogenic Neisseria grow. Vancomycin inhibits gram-positive bacteria, colistin inhibits most gram-negative bacilli and commensal Neisseria, nystatin inhibits yeasts, and trimethoprim inhibits swarming Proteus. Plates are incubated at 35 to 37 °C in 3 to 7% CO2 with humidity. A few N. gonorrhoeae strains are inhibited by vancomycin, so a nonselective chocolate plate is added for specimens from sterile sites.
- A: Nystatin or anisomycin inhibits yeasts and molds.
- C: Colistin, not vancomycin, inhibits gram-negative bacilli.
- D: Trimethoprim inhibits swarming Proteus.
Question 4 · Easy
Under current ACOG guidance, at what point in pregnancy is universal screening for group B streptococcus performed?
1. AAt the first prenatal visit
2. B28 0/7 to 32 0/7 weeks
3. C24 0/7 to 28 0/7 weeks
4. D36 0/7 to 37 6/7 weeks
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Answer: D · 36 0/7 to 37 6/7 weeks
ACOG recommends a vaginal-rectal swab for GBS culture at 36 0/7 to 37 6/7 weeks of gestation, a window moved from the earlier 35 to 37 weeks so that results stay valid for 5 weeks and cover births up to at least 41 weeks. A single swab of the lower vagina and then the rectum is placed in transport medium and enriched in selective broth. Women with a positive screen receive intrapartum antibiotics, usually penicillin. GBS bacteriuria at any time in pregnancy also calls for intrapartum prophylaxis, without the need for screening.
- A: GBS colonization changes during pregnancy, so early screening is not reliable.
- B: A swab at 28 to 32 weeks may not reflect colonization at delivery.
- C: 24 to 28 weeks is the window for gestational diabetes screening, not GBS.
Question 5 · Easy
Clue cells in a vaginal wet mount are most consistent with:
1. AGonococcal cervicitis
2. BTrichomoniasis
3. CBacterial vaginosis
4. DVulvovaginal candidiasis
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Answer: C · Bacterial vaginosis
Clue cells are squamous epithelial cells so heavily coated with small gram-variable coccobacilli that their edges look stippled and blurred. They are one of the Amsel criteria for bacterial vaginosis, together with a thin gray discharge, a vaginal pH above 4.5 and a fishy amine odor with KOH (the whiff test). BV reflects a shift from lactobacilli to Gardnerella vaginalis, Mobiluncus and other anaerobes. Culture for Gardnerella is not recommended, because it is often present in healthy women.
- A: Gonococcal cervicitis is diagnosed by NAAT, not by clue cells.
- B: Trichomoniasis is shown by motile flagellated trichomonads.
- D: Candidiasis is shown by budding yeasts and pseudohyphae.
Question 6 · Medium
In which specimen does a Gram stain showing gram-negative diplococci inside neutrophils allow a presumptive diagnosis of gonorrhea?
1. APharyngeal swab from a man with sore throat
2. BEndocervical swab from a woman with discharge
3. CUrethral discharge from a man with symptoms
4. DRectal swab from a man with proctitis
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Question 7 · Hard
Subculture of a prenatal GBS screening broth grows a non-hemolytic colony of catalase-negative gram-positive cocci: PYR negative, bile esculin negative, CAMP positive, hippurate positive. The correct report is:
1. AGroup B streptococcus detected
2. BViridans streptococci only; no GBS detected
3. CEnterococcus isolated; no GBS detected
4. DNo GBS, as the colony is not beta-hemolytic
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Question 8 · Medium
Which set of findings fits vulvovaginal candidiasis?
1. ApH 5.5, positive whiff test, clue cells present
2. BpH 4.0, lactobacilli only, no yeasts or hyphae
3. CpH 6.0, motile flagellates, many neutrophils
4. DpH 4.2, negative whiff test, pseudohyphae on KOH
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Question 9 · Medium
A pregnant woman with a history of anaphylaxis to penicillin has a positive group B streptococcus screen. The laboratory should perform:
1. APenicillin and ampicillin MIC testing
2. BCefazolin disk diffusion testing
3. CClindamycin testing, including inducible resistance
4. DErythromycin susceptibility testing alone
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Question 10 · Hard
Gram-negative diplococci from a urethral discharge grow on Thayer-Martin agar, are oxidase positive, and produce acid from glucose only (not maltose, lactose or sucrose). What is the organism?
1. ANeisseria meningitidis
2. BMoraxella catarrhalis
3. CNeisseria gonorrhoeae
4. DNeisseria lactamica
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Genital tract

Which test is recommended for Chlamydia trachomatis on a vaginal swab?

A Gram-stained vaginal smear shows few lactobacilli and many small gram-variable rods and curved gram-variable rods. A Nugent score of 8 indicates:

In modified Thayer-Martin agar, which group of organisms does vancomycin inhibit?

Under current ACOG guidance, at what point in pregnancy is universal screening for group B streptococcus performed?

Clue cells in a vaginal wet mount are most consistent with:

In which specimen does a Gram stain showing gram-negative diplococci inside neutrophils allow a presumptive diagnosis of gonorrhea?

Subculture of a prenatal GBS screening broth grows a non-hemolytic colony of catalase-negative gram-positive cocci: PYR negative, bile esculin negative, CAMP positive, hippurate positive. The correct report is:

Which set of findings fits vulvovaginal candidiasis?

A pregnant woman with a history of anaphylaxis to penicillin has a positive group B streptococcus screen. The laboratory should perform:

Gram-negative diplococci from a urethral discharge grow on Thayer-Martin agar, are oxidase positive, and produce acid from glucose only (not maltose, lactose or sucrose). What is the organism?