**Core Concept**
Clue cells are a diagnostic feature seen in bacterial vaginosis, a type of vaginal infection. They are characterized by the presence of bacteria on the surface of vaginal epithelial cells, which alters their morphology. The presence of clue cells is an indicator of an abnormal vaginal flora.

**Why the Correct Answer is Right**
Clue cells are formed when bacteria from the normal vaginal flora, such as Gardnerella vaginalis, adhere to the surface of the vaginal epithelial cells. This adherence causes the cell surface to become irregular and stippled, giving it a distinctive appearance under microscopic examination. The presence of clue cells is often used as a diagnostic criterion for bacterial vaginosis, as it indicates an imbalance in the normal vaginal flora.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because clue cells are not typically seen in cases of trichomoniasis, a different type of vaginal infection caused by the protozoan Trichomonas vaginalis.
**Option B:** This option is incorrect because clue cells are not a characteristic feature of yeast infections, which are typically caused by Candida species.
**Option C:** This option is incorrect because clue cells are not seen in cases of gonorrhea, a sexually transmitted infection caused by the bacterium Neisseria gonorrhoeae.

**Clinical Pearl / High-Yield Fact**
Clue cells are a key diagnostic feature of bacterial vaginosis, and their presence can help differentiate this condition from other types of vaginal infections. It is essential to examine vaginal discharge under a microscope to identify clue cells and make an accurate diagnosis.

**Correct Answer:** B. Bacterial vaginosis.