**Core Concept**
In AIDS patients, the immune system is severely compromised, making them susceptible to opportunistic infections. Acute meningitis in this population is often caused by pathogens that take advantage of this immunocompromised state. The most common cause of acute meningitis in AIDS patients is typically a fungal or viral organism.

**Why the Correct Answer is Right**
Cryptococcal meningitis is the most common cause of acute meningitis in AIDS patients, particularly in those with CD4 counts below 100 cells/μL. This is due to the fact that *Cryptococcus neoformans* is a ubiquitous fungus that can be found in soil and bird droppings. When the immune system is compromised, *C. neoformans* can invade the central nervous system and cause meningitis. The pathogenesis involves the fungal capsule binding to complement receptors on macrophages and subsequent phagocytosis, which allows the fungus to evade the immune response.

**Why Each Wrong Option is Incorrect**
**Option A:** *Escherichia coli* is a common cause of urinary tract infections but is not typically associated with acute meningitis in AIDS patients.
**Option B:** *Haemophilus influenzae* type b is a common cause of meningitis in children but is not typically seen in immunocompromised adults.
**Option C:** *Streptococcus pneumoniae* is a common cause of community-acquired pneumonia and meningitis in healthy individuals but is not the most common cause in AIDS patients.

**Clinical Pearl / High-Yield Fact**
AIDS patients with acute meningitis often present with non-specific symptoms such as headache, fever, and confusion. Imaging studies may show characteristic "punched-out" lesions in the brain, which are indicative of cryptococcal meningitis.

**Correct Answer: C. Cryptococcus neoformans.**