**Core Concept**
The question tests the student's knowledge of the life cycles of various microorganisms, specifically focusing on the presence or absence of a cyst phase. The cyst phase is a dormant, resistant stage that allows certain organisms to survive adverse environmental conditions.

**Why the Correct Answer is Right**
Trichomonas vaginalis is a protozoan parasite that causes trichomoniasis, a sexually transmitted infection. Unlike other protozoa such as Entamoeba species, Trichomonas vaginalis does not have a cyst phase in its life cycle. Instead, it exists in a trophozoite form, which is the active, feeding stage of the organism. This characteristic makes Trichomonas vaginalis unique among the options provided.

**Why Each Wrong Option is Incorrect**
**Option A:** Gardenella vaginalis, now known as Gardnerella vaginalis, is a bacterium, and bacteria do not have a cyst phase in the same way protozoa do, but this is not the best answer choice given the context of the question.
**Option C:** Entamoeba histolytica has a well-defined cyst phase that is essential for its transmission and survival.
**Option D:** Entamoeba coli also has a cyst phase, similar to E. histolytica, which is crucial for its life cycle.

**Clinical Pearl / High-Yield Fact**
Remembering that Trichomonas vaginalis lacks a cyst phase is crucial for understanding its transmission dynamics and why it requires a different approach for diagnosis and treatment compared to organisms with cyst phases.

**Correct Answer:** B. Trichomonas vaginalis