**Core Concept**
The patient's symptoms of chronic abdominal pain, low-grade fever, diarrhea, and malabsorption, combined with the presence of oocysts in the stool, suggest an intestinal infection caused by a protozoan parasite. This scenario is commonly associated with an opportunistic infection in immunocompromised individuals, such as those with AIDS.

**Why the Correct Answer is Right**
The likely cause of the patient's diarrhea is an infection with *Cryptosporidium parvum*, a protozoan parasite that infects the small intestine and causes a self-limiting but severe diarrheal illness in immunocompetent individuals. In immunocompromised patients, such as those with AIDS, *Cryptosporidium* infection can lead to chronic and severe diarrhea, weight loss, and malabsorption. The presence of oocysts in the stool is a diagnostic hallmark of this infection.

**Why Each Wrong Option is Incorrect**
**Option A:** *Giardia lamblia* is another protozoan parasite that can cause diarrhea, but it is more commonly associated with waterborne outbreaks and typically presents with a different clinical picture.

**Option B:** *Isospora belli* is a protozoan parasite that can cause intestinal infection in immunocompromised patients, but it is less commonly associated with chronic diarrhea and malabsorption compared to *Cryptosporidium*.

**Option C:** *Microsporidia* are a group of protozoan parasites that can cause intestinal infection in immunocompromised patients, but they are more commonly associated with HIV/AIDS-related wasting and diarrhea, rather than *Cryptosporidium*.

**Clinical Pearl / High-Yield Fact**
In immunocompromised patients, such as those with AIDS, *Cryptosporidium* infection can be a life-threatening condition, and prompt treatment with antiparasitic agents, such as paromomycin or azithromycin, is essential to prevent severe complications.

**Correct Answer:** D. *Cryptosporidium parvum*