**Core Concept**
The patient's symptoms and laboratory findings are consistent with an opportunistic infection caused by a protozoan parasite, specifically *Cryptosporidium parvum*. This parasite is known to cause cryptosporidiosis, a common cause of diarrhea in immunocompromised patients, including those with AIDS.

**Why the Correct Answer is Right**
The correct answer is based on the fact that *Cryptosporidium parvum* is resistant to many antiparasitic agents, but is highly susceptible to nitazoxanide. Nitazoxanide is an antiprotozoal agent that works by inhibiting the pyruvate:ferredoxin oxidoreductase enzyme of the parasite, leading to the disruption of energy production and ultimately, parasite death. This mechanism of action is unique to nitazoxanide and other related compounds.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because metronidazole is effective against anaerobic bacteria and some protozoa, but it is not the primary treatment for cryptosporidiosis.
**Option B:** This option is incorrect because trimethoprim-sulfamethoxazole is primarily used to treat bacterial infections, including Pneumocystis jirovecii pneumonia, but it is not effective against *Cryptosporidium parvum*.
**Option C:** This option is incorrect because ciprofloxacin is a fluoroquinolone antibiotic that is effective against a wide range of bacterial infections, but it is not the treatment of choice for cryptosporidiosis.
**Option D:** This option is incorrect because albendazole is an anthelmintic agent that is effective against various nematode and cestode infections, but it is not effective against *Cryptosporidium parvum*.

**Clinical Pearl / High-Yield Fact**
Nitazoxanide is a relatively new antiprotozoal agent that has emerged as a treatment option for cryptosporidiosis, a common cause of diarrhea in immunocompromised patients. This drug is particularly useful in patients who are intolerant to other treatments or have failed previous therapies.

**Correct Answer:** C. Nitazoxanide