**Core Concept**
Fecal leukocytes are a useful diagnostic tool for identifying inflammatory diarrhea caused by bacterial pathogens. The presence of leukocytes in stool indicates an inflammatory response, often due to bacterial invasion of the intestinal mucosa. However, not all bacterial infections lead to the presence of fecal leukocytes.

**Why the Correct Answer is Right**
The absence of fecal leukocytes can be seen in viral and parasitic infections, which typically do not cause significant inflammation or bacterial invasion. However, in the case of Giardiasis, a parasitic infection caused by Giardia lamblia, fecal leukocytes are often absent. Giardia attaches to the intestinal mucosa but does not invade the epithelial cells, leading to a non-inflammatory response.

**Why Each Wrong Option is Incorrect**
**Option A:** Campylobacter jejuni typically causes a significant inflammatory response, leading to the presence of fecal leukocytes.
**Option B:** Shigella dysenteriae is a classic example of a bacterial pathogen that causes inflammation and the presence of fecal leukocytes.
**Option C:** Salmonella Typhi also causes an inflammatory response, leading to the presence of fecal leukocytes.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the absence of fecal leukocytes does not rule out a bacterial infection, especially in cases of Giardiasis. A thorough clinical history and laboratory evaluation, including stool cultures and antigen tests, are necessary to diagnose the cause of diarrhea.

**Correct Answer: D. Giardiasis**