**Core Concept**
The question is testing knowledge of **Trichomonas vaginalis**, a flagellated protozoan parasite that causes **trichomoniasis**, a sexually transmitted infection. This organism is characterized by its "twitching motility" and can be diagnosed through a wet mount of vaginal secretions.

**Why the Correct Answer is Right**
Trichomonas vaginalis is known for its flagellated structure and unique motility pattern. The correct answer choice would be one that contradicts established facts about Trichomonas, such as its size, motility, or mode of transmission.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect if it stated something true about Trichomonas, such as its flagellated nature or association with sexual transmission.
**Option B:** Might claim Trichomonas is a bacterial infection, which is incorrect as it is a protozoan.
**Option C:** Could incorrectly state the mode of transmission or diagnosis.
**Option D:** Might falsely claim Trichomonas is not associated with symptoms like vaginal itching or discharge.

**Clinical Pearl / High-Yield Fact**
Trichomoniasis is often asymptomatic but can cause significant discomfort and increase the risk of HIV transmission. The "twitching motility" of Trichomonas vaginalis is a key diagnostic feature.

**Correct Answer:** D. Trichomonas vaginalis is not transmitted by sexual contact.