A3-week old child presented to the pediatrician with meningitis. A presumptive diagnosis of late onset of a perinatal infection was made. The CSF culture was positive for gram positive bacilli. Which of the following characteristic of this bacteria would be helpful in differentiating it from other bacteria agents –

**Core Concept**
The question is testing the ability to identify a key characteristic of a specific type of bacteria that causes meningitis in infants, particularly in the context of late-onset perinatal infection. The underlying concept involves the differentiation of gram-positive bacilli based on their unique features.

**Why the Correct Answer is Right**
The correct answer involves the identification of a characteristic that helps differentiate the bacteria from other gram-positive bacilli. In this case, the bacteria are likely *Listeria monocytogenes*, which is a common cause of meningitis in infants, especially in the context of late-onset perinatal infection. *Listeria monocytogenes* is known for its ability to grow at refrigerated temperatures and is a facultative intracellular pathogen. This characteristic is essential in differentiating it from other gram-positive bacilli.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not provide a specific characteristic that can be used to differentiate the bacteria from other gram-positive bacilli. The ability to grow at high temperatures is not a unique feature of *Listeria monocytogenes*.
* **Option B:** This option is incorrect because it mentions the presence of a capsule, which is a feature shared by many gram-positive bacilli. While the capsule is an important virulence factor, it is not a unique characteristic that can be used to differentiate *Listeria monocytogenes* from other bacteria.
* **Option C:** This option is incorrect because it suggests that the bacteria are motile, which is not a characteristic of *Listeria monocytogenes*. *Listeria monocytogenes* is a non-motile organism.

**Clinical Pearl / High-Yield Fact**
* *Listeria monocytogenes* is a gram-positive bacillus that is known for its ability to grow at refrigerated temperatures (4°C) and is a facultative intracellular pathogen. This characteristic is essential in differentiating it from other gram-positive bacilli.

**Correct Answer:** D.

✓ Correct Answer: D. Motility at 25°C