**Core Concept**
Bacterial vaginosis (BV) is a type of vaginal infection caused by an imbalance of the normal microbial flora in the vagina, characterized by an overgrowth of various anaerobic bacteria. The exact pathogenesis of BV is complex and involves multiple factors, including hormonal changes, altered vaginal pH, and impaired immune response.

**Why the Correct Answer is Right**
BV is caused by an overgrowth of anaerobic bacteria, including Gardnerella vaginalis, Bacteroides, and Peptostreptococcus species. These bacteria produce enzymes such as aminohydrolase and aminopeptidase, which break down the lactic acid produced by Lactobacillus species, leading to an increase in vaginal pH and the characteristic symptoms of BV. The pathogenesis of BV is also influenced by hormonal changes, particularly the decrease in progesterone levels, which allows the anaerobic bacteria to overgrow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because BV is not caused by a single pathogen, but rather by an imbalance of the normal vaginal flora.
**Option B:** This option is incorrect because the primary cause of BV is not the decrease in estrogen levels, but rather the overgrowth of anaerobic bacteria.
**Option C:** This option is incorrect because BV is not a sexually transmitted infection, although it can be transmitted during sexual contact.

**Clinical Pearl / High-Yield Fact**
BV is a significant risk factor for preterm labor, low birth weight, and pelvic inflammatory disease. Therefore, it is essential to diagnose and treat BV promptly in pregnant women and those with a history of pelvic inflammatory disease.

**Correct Answer: D.**