**Core Concept**
Bacterial vaginosis (BV) is a type of vaginal infection characterized by an imbalance of the normal vaginal flora, leading to an overgrowth of various bacteria. The diagnosis of BV is typically made based on clinical symptoms, such as vaginal discharge and odor, and is supported by laboratory findings, including microscopy of vaginal discharge.

**Why the Correct Answer is Right**
The diagnosis of BV by microscopy of vaginal discharge is supported by the presence of clue cells, which are vaginal epithelial cells that are covered in bacteria, particularly Gardnerella vaginalis. Clue cells are identified by their characteristic appearance, with the bacteria covering the cell surface, making it difficult to see the cell borders. This is often confirmed by the addition of a Gram stain, which highlights the bacteria.

**Why Each Wrong Option is Incorrect**
* **Option A:** The presence of lactobacilli in the vaginal discharge is actually a normal finding and is not indicative of BV.
* **Option B:** The presence of white blood cells in the vaginal discharge may be seen in other types of vaginal infections, such as yeast infections or trichomoniasis, but is not a specific indicator of BV.
* **Option D:** The presence of a thick, white, cottage cheese-like discharge is more characteristic of a yeast infection, such as candidiasis, rather than BV.

**Clinical Pearl / High-Yield Fact**
To diagnose BV by microscopy, it's essential to look for clue cells, which are vaginal epithelial cells with a characteristic "stippled" appearance due to the overlying bacteria.

**Correct Answer:** A.