**Core Concept**
Bacterial vaginosis (BV) is a polymicrobial vaginal infection characterized by an imbalance of the normal vaginal microbiota, leading to an overgrowth of anaerobic bacteria. This condition is distinct from other vaginal infections such as candidiasis and trichomoniasis.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of bacterial vaginosis. BV is caused by an increase in the pH of the vagina due to the overproduction of amines by anaerobic bacteria, such as Gardnerella vaginalis. This increase in pH disrupts the normal acidic environment of the vagina, leading to symptoms such as vaginal discharge and odor. The diagnosis of BV is typically made based on the presence of a "fishy" odor when a potassium hydroxide (KOH) solution is added to the vaginal discharge.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because bacterial vaginosis is often associated with an increase in pH, not a decrease. The alkalinity of the vagina promotes the growth of anaerobic bacteria, which is a hallmark of BV.

**Option B:** This option is incorrect because the primary symptom of bacterial vaginosis is not itching, but rather a thin, white or gray vaginal discharge with a "fishy" odor. While itching may occur, it is not a characteristic symptom of BV.

**Option C:** This option is incorrect because the diagnosis of bacterial vaginosis is typically made based on the presence of a "fishy" odor when a potassium hydroxide (KOH) solution is added to the vaginal discharge, not a Gram stain.

**Clinical Pearl / High-Yield Fact**
A classic clinical pearl for diagnosing bacterial vaginosis is the "Amsel criteria", which includes four major criteria: a thin, white or gray vaginal discharge, a "fishy" odor when a potassium hydroxide (KOH) solution is added to the discharge, a pH of the vagina greater than 4.5, and the presence of clue cells on a wet mount.

**Correct Answer: D.**