## Core Concept
Meningitis caused by gram-positive bacilli is a serious condition that requires prompt identification and treatment. Gram-positive bacilli are a diverse group of bacteria, and not all species are commonly associated with meningitis. The most notable gram-positive bacillus known to cause meningitis is *Listeria monocytogenes*.

## Why the Correct Answer is Right
*Listeria monocytogenes* is a gram-positive rod that is known to cause severe infections, particularly in immunocompromised individuals, pregnant women, and neonates. It is one of the few gram-positive bacilli that can cause meningitis. *Listeria* species are facultative intracellular pathogens that can cross the blood-brain barrier, leading to central nervous system infections, including meningitis.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because *Bacillus* species, although gram-positive bacilli, are not commonly associated with meningitis. Some *Bacillus* species can cause infections in humans, but they are more commonly known for causing food poisoning or being contaminants.
* **Option B:** This option is incorrect as it is not specified, but if it refers to other gram-positive bacilli not commonly associated with meningitis, they would not be the correct answer for the same reason *Listeria* is the correct one.
* **Option C:** Without a specific organism listed, it's hard to directly address, but generally, other gram-positive bacilli are not commonly implicated in meningitis.
* **Option D:** This option being incorrect implies that another organism not commonly associated with gram-positive bacilli meningitis or not specified would not be correct.

## Clinical Pearl / High-Yield Fact
A key point to remember is that *Listeria monocytogenes* is a cause of meningitis, particularly in specific populations such as the elderly, neonates, pregnant women, and immunocompromised individuals. When treating suspected bacterial meningitis, coverage for *Listeria* should be considered in these high-risk groups.

## Correct Answer: C. Listeria monocytogenes