**Core Concept**
The question is testing the identification of an acid-fast organism causing diarrhea in an immunocompromised patient, specifically an HIV-positive individual. This organism is known to have a large oocyte size, which is a key distinguishing characteristic.

**Why the Correct Answer is Right**
The organism in question is *Cryptosporidium parvum*, a protozoan parasite that is acid-fast and causes cryptosporidiosis. This infection is particularly severe in immunocompromised patients, such as those with HIV/AIDS, leading to life-threatening diarrhea and malabsorption. The large oocyte size, approximately 5-6 μm in diameter, is a characteristic feature of *C. parvum*.

**Why Each Wrong Option is Incorrect**
* **Option A:** *Giardia lamblia* is a different protozoan parasite that can cause gastrointestinal infections, but it is not acid-fast and has a smaller oocyte size (8-12 μm in diameter).
* **Option B:** *E. histolytica* is a protozoan parasite that can cause amoebic dysentery, but it is not acid-fast and has a different oocyte morphology.
* **Option C:** *Isospora belli* is another acid-fast protozoan parasite that can cause intestinal infections in immunocompromised patients, but its oocyte size is smaller (25-30 μm in diameter).
* **Option D:** *Microsporidia* are a group of obligate intracellular parasites that can cause microsporidiosis, but they are not typically acid-fast and have a different oocyte morphology.

**Clinical Pearl / High-Yield Fact**
In immunocompromised patients, particularly those with HIV/AIDS, a large oocyte size on stool examination should raise suspicion for *Cryptosporidium parvum* infection, which can be treated with antiparasitic medications such as azithromycin or paromomycin.

**Correct Answer:** C. *Isospora belli*.