## Core Concept
The question revolves around the identification of a gram-positive bacillus causing late-onset perinatal meningitis in a 3-week-old child. The key here is to recognize the common pathogens responsible for neonatal meningitis and their characteristics. **Listeria monocytogenes** is a notorious cause of late-onset neonatal meningitis and is known for being a gram-positive bacillus.

## Why the Correct Answer is Right
The correct answer, , is indicative of **Listeria monocytogenes** due to its characteristic of being **catalase-negative** or having a variable catalase reaction but more importantly, its ability to form **tumbling motility** at room temperature (20-25°C) and its **positive bile solubility test**. However, the most distinctive feature among the given options related to *Listeria* would be its tumbling motility. This differentiates it from other gram-positive bacilli like *Bacillus* and *Corynebacterium* species.

## Why Each Wrong Option is Incorrect
- **Option A:** This option does not provide enough information to directly link it to a distinguishing feature of *Listeria* or other relevant gram-positive bacilli causing meningitis.
- **Option B:** Similarly, without specifics, it's hard to directly refute, but common distinguishing features like spore formation or specific types of hemolysis are not mentioned.
- **Option C:** This could relate to several bacteria and does not specifically point towards a characteristic unique to the likely pathogen in this scenario.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **Listeria monocytogenes** is a significant cause of neonatal meningitis, particularly the late-onset form (occurring after 1 week of life), and is often associated with outbreaks of foodborne illness. A high index of suspicion is necessary for early diagnosis, especially in pregnant women and newborns.

## Correct Answer: D.