**Core Concept**
Giardiasis is an intestinal infection caused by the protozoan parasite Giardia lamblia, characterized by the presence of mobile trophozoites in the stool without the presence of blood or pus cells. Giardia lamblia is a flagellated protozoan that adheres to the intestinal mucosa, leading to malabsorption and diarrhea.

**Why the Correct Answer is Right**
The diagnosis of giardiasis is based on the identification of Giardia lamblia trophozoites in the stool, which are typically mobile and have a distinctive pear-shaped appearance. The absence of red blood cells (RBCs) and pus cells in the stool differentiates giardiasis from other intestinal infections such as amoebic dysentery caused by Entamoeba histolytica. Giardia lamblia trophozoites adhere to the intestinal mucosa using a unique mechanism that involves the expression of specific adhesins and the presence of a ventral disc.

**Why Each Wrong Option is Incorrect**
**Option A:** Balantidium coli is a larger protozoan parasite that can cause dysentery, characterized by the presence of blood and pus cells in the stool, which is not consistent with the given scenario.
**Option C:** Trichomonas hominis is a flagellated protozoan that is typically found in the human mouth, throat, or gastrointestinal tract, but it is not commonly associated with diarrhea.
**Option D:** Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery, which is characterized by the presence of blood and pus cells in the stool, along with the presence of cysts in the stool.

**Clinical Pearl / High-Yield Fact**
Giardiasis is often acquired through contaminated water or food, and it can be transmitted through person-to-person contact, particularly in childcare settings. The diagnosis of giardiasis can be challenging, and it often requires the examination of multiple stool specimens.

✓ Correct Answer: B. Giardiasis