Bacterial Vaginosis and Trichomoniasis are easy to mix up on the boards. Here's a side-by-side comparison — presentation, workup, imaging, and first-line treatment — drawn from our full outlines.
Bacterial Vaginosis vs Trichomoniasis at a glance
Bacterial Vaginosis: Dysbiosis of vaginal flora — loss of lactobacilli with overgrowth of anaerobes; thin gray discharge with fishy odor.
Try two board-style questions on Bacterial Vaginosis vs Trichomoniasis
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Question 1ReproductiveMedium
A 27-year-old woman presents with a 2-week history of a thin gray-white vaginal discharge and a fishy odor that worsens after intercourse. She reports no vulvar itching or burning. Pelvic examination reveals a homogeneous discharge coating the vaginal walls without erythema or cervical friability. Vaginal pH is 5.0. A sample of the discharge is prepared for microscopy. Which of the following findings best supports the diagnosis?
AAbundant neutrophils with intracellular gram-negative diplococci present
BEpithelial cells coated with adherent bacteria obscuring their borders
CMotile flagellated protozoa swimming actively on a saline wet mount
DBranching pseudohyphae and budding yeast on potassium hydroxide prep
Reveal answer & full explanation
Correct answer: B — Epithelial cells coated with adherent bacteria obscuring their borders
AAbundant neutrophils with intracellular gram-negative diplococci present
BEpithelial cells coated with adherent bacteria obscuring their borders✓
CMotile flagellated protozoa swimming actively on a saline wet mount
DBranching pseudohyphae and budding yeast on potassium hydroxide prep
Why Epithelial cells coated with adherent bacteria obscuring their borders is correct
These are clue cells — vaginal epithelial cells so densely coated with adherent coccobacilli that their borders become indistinct. They are the microscopic hallmark of bacterial vaginosis.
BV is diagnosed clinically by the Amsel criteria (3 of 4): thin homogeneous gray-white discharge, vaginal pH >4.5, positive whiff (amine) test with KOH, and clue cells on saline wet mount. This patient already has the discharge, an elevated pH of 5.0, and the odor, so clue cells confirm the diagnosis.
The wet mount in BV also shows few lactobacilli and an absence of WBCs, reflecting overgrowth of anaerobes without an inflammatory response.
Why the others are wrong
Branching pseudohyphae and budding yeast on potassium hydroxide prep — this is the finding in vulvovaginal candidiasis, which presents with thick white "cottage cheese" discharge, intense pruritus, vulvar erythema, and a normal pH <4.5, not the thin gray odorous discharge with pH 5.0 seen here.
Motile flagellated protozoa swimming actively on a saline wet mount — motile trichomonads indicate trichomoniasis, which causes a frothy yellow-green discharge, a strawberry cervix, and pH >4.5; the discharge here is gray-white and homogeneous without those features.
Abundant neutrophils with intracellular gram-negative diplococci present — this points to gonococcal cervicitis, which produces mucopurulent cervical discharge and friability with many WBCs; BV characteristically lacks neutrophils and cervical friability.
Question 2ReproductiveMedium
A 26-year-old woman presents with 5 days of frothy, malodorous yellow-green vaginal discharge, vulvar itching, and burning with urination. She is not pregnant and has had two new partners in the past 2 months. On speculum exam there is vaginal erythema and punctate cervical hemorrhages. Vaginal pH is 5.5, and saline wet mount shows motile, pear-shaped flagellated organisms. Which of the following is the most appropriate initial management?
AMetronidazole 250 mg orally three times daily for 7 days
BMetronidazole 500 mg orally twice daily for 7 days
CMetronidazole 2 g orally as a single one-time dose
DMetronidazole 0.75% vaginal gel nightly for 5 days
Reveal answer & full explanation
Correct answer: B — Metronidazole 500 mg orally twice daily for 7 days
AMetronidazole 250 mg orally three times daily for 7 days
BMetronidazole 500 mg orally twice daily for 7 days✓
CMetronidazole 2 g orally as a single one-time dose
DMetronidazole 0.75% vaginal gel nightly for 5 days
Why Metronidazole 500 mg orally twice daily for 7 days is correct
The frothy yellow-green discharge, strawberry cervix (punctate hemorrhages), pH >4.5, and motile flagellated trichomonads on wet mount confirm Trichomonas vaginalis vaginitis.
Per CDC 2021 STI treatment guidelines, first-line therapy for women is metronidazole 500 mg PO BID for 7 days; this multidose regimen replaced the single 2 g dose in women because the 7-day course produces lower recurrence.
Sexual partners should also be treated, and the patient should be re-tested within 3 months given high reinfection rates.
Why the others are wrong
Metronidazole 2 g orally as a single one-time dose — correct drug class, but the single dose is now first-line only for men; in women it has higher recurrence and is no longer preferred.
Metronidazole 250 mg orally three times daily for 7 days — a superseded lower-dose oral regimen that delivers less total drug than the currently recommended course; CDC specifies 500 mg twice daily for 7 days in women.
Metronidazole 0.75% vaginal gel nightly for 5 days — topical metronidazole treats bacterial vaginosis but does not reach therapeutic levels in the urethra and periurethral glands where trichomonads persist, so cure rates are poor and it is explicitly not recommended for trichomoniasis.
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Reproductive-age woman with thin gray discharge, fishy odor after intercourse, vaginal pH >4.5, clue cells on wet mount, positive whiff test.; Thin, gray-white homogeneous vaginal discharge; Fishy odor, often worse after intercourse or menses; Minimal or no pruritus or inflammation (distinguishes from candidiasis/trichomoniasis); ~50%…
Sexually active woman with frothy malodorous discharge, vulvar pruritus, and strawberry cervix on colposcopy or speculum exam.; Frothy, malodorous yellow-green discharge; Vulvar and vaginal pruritus, burning; Dyspareunia, dysuria; Postcoital bleeding; ~50% of women and most men are asymptomatic; Frothy yellow-green discharge;…
Workup / key labs
Amsel criteria (3 of 4): (1) thin homogeneous gray-white discharge, (2) vaginal pH >4.5, (3) positive whiff test with KOH, (4) clue cells on saline wet mount. Nugent score (gram-stain based) is research/lab gold standard.; Vaginal pH (normal 3.8-4.5; BV >4.5); Wet mount: clue cells (epithelial cells coated with bacteria obscuring…
NAAT (nucleic acid amplification test) — preferred; high sensitivity/specificity; Wet mount microscopy — motile, pear-shaped flagellated trichomonads (~50-70% sensitivity); Point-of-care antigen tests (OSOM Trichomonas, Affirm VPIII) — better than wet mount, suitable for clinical setting; Culture (Diamond medium) — high specificity but…
First-line treatment
Metronidazole 500 mg PO BID × 7 days; Metronidazole 0.75% vaginal gel 5 g intravaginally daily × 5 days; Clindamycin 2% vaginal cream 5 g intravaginally at bedtime × 7 days
Women: metronidazole 500 mg PO BID × 7 days (CDC 2021 — replaced single 2 g dose for women based on trial data showing lower recurrence with 7-day regimen); Men: metronidazole 2 g PO × 1 dose; Alternative: tinidazole 2 g PO × 1 dose
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