**Core Concept**
In immunocompromised individuals, such as those with AIDS, various opportunistic infections can occur due to the compromised immune system's inability to fight off pathogens. Parasitic infections are a significant concern in these patients.

**Why the Correct Answer is Right**
The most common parasitic infection in AIDS patients is *Toxoplasma gondii*. This is because the immunocompromised state of AIDS patients allows *T. gondii* to reactivate from latent infection, leading to toxoplasmosis. Toxoplasmosis can cause serious neurological complications in these patients, including encephalitis.

**Why Each Wrong Option is Incorrect**
**Option A:** *Cryptococcus neoformans* is a fungal pathogen, not a parasite. It is also a common opportunistic infection in AIDS patients but is not a parasitic infection.
**Option B:** *Strongyloides stercoralis* is a parasitic nematode (roundworm) but is not the most common parasitic infection in AIDS patients. It can cause strongyloidiasis, which can be severe in immunocompromised individuals.
**Option C:** *Leishmania* is a parasitic protozoan that causes leishmaniasis, a disease more commonly seen in travelers to endemic areas and not typically the most common parasitic infection in AIDS patients.
**Option D:** *Plasmodium falciparum* is the parasite responsible for malaria, but it is not the most common parasitic infection in AIDS patients.

**Clinical Pearl / High-Yield Fact**
In immunocompromised patients, including those with AIDS, it is essential to consider opportunistic infections, such as toxoplasmosis, when evaluating neurological symptoms.

**Correct Answer: B. Strongyloides stercoralis**