**Core Concept**
Bacterial vaginosis (BV) is a type of vaginal infection caused by an imbalance of the normal vaginal flora, characterized by a decrease in the levels of lactobacilli and an overgrowth of various anaerobic bacteria, including Gardnerella vaginalis.

**Why the Correct Answer is Right**
Bacterial vaginosis is a polymicrobial infection, meaning it involves multiple types of bacteria. The primary cause is an overgrowth of anaerobic bacteria, such as Gardnerella vaginalis, Peptostreptococcus, and Mobiluncus species, which disrupt the normal vaginal flora dominated by lactobacilli. This imbalance leads to an increase in vaginal pH, producing characteristic symptoms and signs.

**Why Each Wrong Option is Incorrect**
**Option A:** Vaginal yeast infections are caused by an overgrowth of Candida species, not bacterial vaginosis. Candida infections have distinct symptoms and are treated differently.

**Option B:** Trichomoniasis is a different type of vaginal infection, caused by the protozoan Trichomonas vaginalis, and is not a type of bacterial vaginosis.

**Option C:** Group B streptococcal infections are a type of bacterial infection that can affect pregnant women and newborns, but are not associated with bacterial vaginosis.

**Clinical Pearl / High-Yield Fact**
Bacterial vaginosis is often associated with an increased risk of preterm labor, low birth weight, and pelvic inflammatory disease (PID).

**Correct Answer: B. Trichomoniasis. Trichomoniasis.**