## **Core Concept**
The question tests the ability to diagnose a diarrheal illness based on stool wet mount findings, focusing on the identification of a protozoan cause. The presence of mobile protozoa without red blood cells (RBCs) and pus cells in the stool suggests an infection caused by a protozoan organism that does not invade the intestinal mucosa.

## **Why the Correct Answer is Right**
The correct answer, **Giardia lamblia**, is a flagellated protozoan that causes giardiasis, characterized by diarrhea, abdominal pain, and sometimes malabsorption. Giardiasis typically presents with a watery diarrhea without blood or pus. The organisms are often seen as pear-shaped or oval, flagellated forms on wet mount. Giardia does not invade the intestinal mucosa; hence, stool examinations usually do not show RBCs or pus cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Entamoeba histolytica* causes amoebic dysentery, which typically presents with bloody diarrhea and the presence of RBCs and pus cells in the stool due to its invasive nature into the intestinal mucosa.
- **Option B:** *Cryptosporidium* can cause diarrhea without blood or pus, but it is usually identified using acid-fast staining or immunoassays on stool rather than wet mount, as it is an intracellular organism.
- **Option C:** *Cyclospora* also causes diarrhea and can be identified in stool but often requires special staining (e.g., acid-fast) for detection.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that giardiasis can be associated with travel or exposure to contaminated water, and it often causes a foul-smelling, greasy diarrhea. A wet mount for giardiasis is best performed with a warm saline solution to visualize the trophozoites' characteristic motility.

## **Correct Answer:** D. Giardia lamblia