A 35-year-old woman complains of vaginal discomfort for 2 weeks. Physical examination reveals a scanty vaginal discharge. The fluid develops a “fishy” odor after treatment with 10% potassium hydroxide. A Pap smear taken during the pelvic examination shows squamous cells covered by coccobacilli (“clue” cells). Which of the following is the most likely etiology of vaginal discomfort in this patient?

**Core Concept**
The patient's symptoms and laboratory findings are consistent with bacterial vaginosis (BV), a common vaginal infection characterized by an imbalance of the normal vaginal microbiota. BV is associated with an overgrowth of anaerobic bacteria, particularly those belonging to the genus Gardnerella.

**Why the Correct Answer is Right**
The presence of "clue" cells (squamous cells covered by coccobacilli) and a "fishy" odor after treatment with potassium hydroxide are diagnostic hallmarks of bacterial vaginosis. The loss of the normal vaginal acidic pH and the overgrowth of anaerobic bacteria lead to the formation of volatile sulfur compounds, responsible for the characteristic odor. Gardnerella vaginalis is a key organism involved in the pathogenesis of BV, and its presence is often confirmed by Gram stain or molecular testing.

**Why Each Wrong Option is Incorrect**
**Option A:** Trichomonas vaginalis is a sexually transmitted protozoan parasite that causes trichomoniasis, a different type of vaginal infection. Although it can cause vaginal discharge and discomfort, it does not produce a "fishy" odor after potassium hydroxide treatment.
**Option B:** Candida species are fungi that cause vulvovaginal candidiasis, a common type of vaginal yeast infection. Candida infections typically present with a thick, white, cottage cheese-like discharge and are not associated with a "fishy" odor.
**Option C:** Neisseria gonorrhoeae is a sexually transmitted bacterium that causes gonorrhea, a different type of vaginal infection. Gonorrhea typically presents with a purulent discharge and is not associated with a "fishy" odor after potassium hydroxide treatment.

**Clinical Pearl / High-Yield Fact**
Bacterial vaginosis is often asymptomatic and may not be diagnosed until it leads to complications, such as preterm labor or pelvic inflammatory disease. A high index of suspicion and prompt diagnosis are essential to prevent these complications.

**Correct Answer:** D. Gardnerella vaginalis

✓ Correct Answer: B. Gardnerella vaginalis