A 26-year-old lady, complains to her gynaecologist of discomfo during intercourse. Pelvic examination demonstrates a frothy, yellow-green vaginal discharge with a strong odour and small, red, ulcerations of the vaginal wall. A wet mount preparation demonstrates motile, flagellated protozoa. Which of the following is the most likely causative organism?

**Core Concept**
Trichomonas vaginalis is a sexually transmitted protozoan infection that causes vaginitis in women, characterized by a frothy, yellow-green vaginal discharge with a strong odor and small, red ulcerations of the vaginal wall.

**Why the Correct Answer is Right**
Trichomonas vaginalis is a flagellated protozoan that infects the urogenital tract, leading to inflammation and irritation of the vaginal mucosa. The infection is associated with the production of copious, frothy discharge due to the release of enzymes that break down the vaginal epithelial cells. The presence of motile, flagellated protozoa on a wet mount preparation is a diagnostic feature of Trichomonas vaginalis infection. The organism adheres to the vaginal epithelial cells via its surface lectins, leading to inflammation and ulceration of the vaginal wall.

**Why Each Wrong Option is Incorrect**
**Option A:** Giardia lamblia is a flagellated protozoan that infects the small intestine, causing giardiasis, and is not associated with vaginal discharge or ulcerations.

**Option C:** Candida albicans is a fungus that causes vaginal candidiasis, characterized by a thick, white, cottage cheese-like discharge, and is not associated with motile protozoa on a wet mount preparation.

**Option D:** Entamoeba histolytica is an amoeba that causes amoebic dysentery, primarily affecting the large intestine, and is not associated with vaginal discharge or ulcerations.

**Clinical Pearl / High-Yield Fact**
Trichomonas vaginalis is a common sexually transmitted infection that can be diagnosed using a wet mount preparation, and treatment typically involves metronidazole or tinidazole.

**✓ Correct Answer: B. Trichomonas vaginalis**

✓ Correct Answer: B. Trichomonas vaginalis