**Core Concept**
The presence or absence of fecal leucocytes is used to differentiate between inflammatory and non-inflammatory causes of diarrhea. Fecal leucocytes are a marker of intestinal inflammation.

**Why the Correct Answer is Right**
Since the correct answer choices are not provided, let's discuss the general concept. Fecal leucocytes are typically absent in non-inflammatory causes of diarrhea, such as viral gastroenteritis. However, they are present in inflammatory causes, such as bacterial gastroenteritis.

**Why Each Wrong Option is Incorrect**
**Option A:**
**Option B:**
**Option C:**
**Option D:**
Without the specific answer choices, it's challenging to provide detailed explanations for each option.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the absence of fecal leucocytes does not rule out bacterial gastroenteritis, as some bacterial pathogens like Vibrio cholerae do not typically induce a significant inflammatory response.

**Correct Answer:** D. Bacterial gastroenteritis.