**Core Concept**
Bacterial vaginosis (BV) is a common vaginal infection caused by an imbalance of the normal vaginal microbiota, leading to an overgrowth of anaerobic bacteria. The diagnosis of BV is typically made based on clinical criteria and microscopy findings.

**Why the Correct Answer is Right**
The diagnosis of BV is primarily based on the presence of clue cells, which are vaginal epithelial cells coated with bacteria, and an abnormal vaginal pH. Clue cells are identified by microscopy, and their presence is a key diagnostic criterion for BV. The presence of clue cells indicates that the normal vaginal flora has been disrupted, allowing anaerobic bacteria to overgrow.

**Why Each Wrong Option is Incorrect**
**Option A:** There is no specific mention of lactobacilli in the question, but in the context of BV, lactobacilli are often reduced or absent. However, lactobacilli are not a diagnostic criterion for BV, and their presence or absence is not typically used to exclude the diagnosis.

**Option B:** The presence of white blood cells (leukocytes) in the vaginal discharge may indicate an inflammatory response, but it is not a specific criterion for diagnosing BV. BV is characterized by a lack of inflammation, and the presence of leukocytes may suggest a different condition, such as trichomoniasis or yeast infection.

**Option C:** A high vaginal pH is a characteristic finding in BV, as the normal vaginal pH is acidic due to the presence of lactobacilli. An elevated pH indicates that the normal vaginal flora has been disrupted, allowing anaerobic bacteria to overgrow.

**Clinical Pearl / High-Yield Fact**
To remember the key diagnostic criterion for BV, recall the phrase "Amsel's criteria": a thin, white or gray vaginal discharge, a pH of 4.5 or greater, the presence of clue cells, a fishy odor, and a positive whiff test.

**Correct Answer:** B.