**Core Concept**
Metachromatic granules are cytoplasmic inclusions found in certain bacteria, which exhibit a distinct color change when stained with specific dyes, such as methylene blue or crystal violet. These granules are composed of lipopolysaccharides and are a characteristic feature of Gram-negative bacteria.

**Why the Correct Answer is Right**
The correct answer is related to the bacterium *Corynebacterium diphtheriae*. Metachromatic granules are a distinctive feature of this bacterium, which is responsible for diphtheria. The granules are formed due to the accumulation of phospholipids and other lipids in the bacterial cytoplasm. When stained with methylene blue, these granules exhibit a characteristic metachromatic effect, taking on a purple color.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because metachromatic granules are not a characteristic feature of *Staphylococcus aureus*. While *S. aureus* can exhibit various cytoplasmic inclusions, metachromatic granules are not one of them.
* **Option B:** This option is incorrect because *Escherichia coli* does not typically exhibit metachromatic granules. While *E. coli* can form various types of inclusions, metachromatic granules are not a characteristic feature of this bacterium.
* **Option C:** This option is incorrect because *Pseudomonas aeruginosa* does not exhibit metachromatic granules. While *P. aeruginosa* can form biofilms and exhibit various virulence factors, metachromatic granules are not a characteristic feature of this bacterium.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that metachromatic granules are a key diagnostic feature of *Corynebacterium diphtheriae*, and their presence can help differentiate this bacterium from other Gram-positive rods.

**Correct Answer:** C. Corynebacterium diphtheriae.