**Core Concept**
The patient's symptoms and stool microscopy findings suggest an infection caused by a protozoan parasite, specifically *Giardia lamblia*. This parasite is a common cause of gastrointestinal infections worldwide, leading to symptoms such as diarrhea, vomiting, and abdominal pain.

**Why the Correct Answer is Right**
*Giardia lamblia* infection occurs when the parasite attaches to the intestinal mucosa, leading to malabsorption of nutrients and electrolytes. The treatment of choice for giardiasis is metronidazole, which works by inhibiting the pyruvate ferredoxin oxidoreductase enzyme in the parasite's mitochondria, ultimately leading to the death of the parasite.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because paromomycin is an antibiotic that is effective against bacterial and protozoan infections, but it is not the first-line treatment for giardiasis.
* **Option B:** This option is incorrect because albendazole is a broad-spectrum anthelmintic that is effective against various helminthic infections, but it is not the preferred treatment for giardiasis.
* **Option C:** This option is incorrect because fluconazole is an antifungal agent that is effective against fungal infections, but it is not used to treat giardiasis.

**Clinical Pearl / High-Yield Fact**
One important clinical correlation to remember is that *Giardia lamblia* infection can be transmitted through contaminated water or food, and it is often seen in individuals who have recently traveled to areas with poor sanitation. This is a classic example of a "traveler's diarrhea" case.

**Correct Answer:** C.