**Core Concept**
In patients with diarrhea, the presence or absence of leukocytes (white blood cells) in the stool can help differentiate between infectious and non-infectious causes. A negative fecal smear for leukocytes typically suggests a non-inflammatory or non-infectious cause of diarrhea.

**Why the Correct Answer is Right**
The absence of leukocytes in the stool suggests that the diarrhea is not due to an invasive bacterial or viral infection that would typically trigger an inflammatory response. In this case, the most likely cause of diarrhea is a non-infectious, non-inflammatory condition such as food poisoning from a toxin-producing organism like Staphylococcus aureus or Bacillus cereus. These organisms produce heat-stable enterotoxins that cause diarrhea without invading the intestinal mucosa or triggering an inflammatory response.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because invasive bacterial infections like Shigella or Salmonella typically cause a fecal smear positive for leukocytes due to the inflammatory response they trigger.
* **Option B:** This option is incorrect because viral infections like rotavirus or norovirus can cause diarrhea with a negative fecal smear, but they are not the most likely cause in this case.
* **Option D:** This option is incorrect because inflammatory bowel diseases like Crohn's disease or ulcerative colitis typically cause a fecal smear positive for leukocytes due to the chronic inflammation in the gut.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with diarrhea, remember that a negative fecal smear for leukocytes suggests a non-inflammatory or non-infectious cause, and a positive fecal smear suggests an infectious or inflammatory cause.

**Correct Answer:** C.