## **Core Concept**
The question tests knowledge of bacterial cell wall structure and the Gram staining technique. Gram-positive bacteria retain the crystal violet stain used in the Gram staining procedure due to their thick **peptidoglycan layer**. This layer is located outside the bacterial cell membrane and provides structural support.

## **Why the Correct Answer is Right**
The correct answer, **Staphylococcus aureus**, is a Gram-positive bacterium. This classification is due to its cell wall composition, which features a thick peptidoglycan layer. When subjected to Gram staining, **Staphylococcus aureus** appears purple under a microscope because it retains the crystal violet stain. This characteristic is a defining feature of Gram-positive bacteria.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Escherichia coli* is a Gram-negative bacterium. Its cell wall contains a thinner peptidoglycan layer and an outer membrane with **lipopolysaccharides**, which causes it to appear pink or red under Gram staining due to the counterstain.
- **Option B:** *Klebsiella pneumoniae* is also a Gram-negative bacterium. Similar to *Escherichia coli*, its cell wall structure leads to Gram-negative staining characteristics.
- **Option C:** *Pseudomonas aeruginosa* is a Gram-negative bacterium as well. It has a thin peptidoglycan layer and an outer membrane, which results in a Gram-negative staining pattern.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Gram-positive bacteria, such as **Staphylococcus aureus**, are more likely to cause skin and soft tissue infections, as well as infections related to implanted devices, due to their ability to form biofilms and produce virulence factors like **coagulase**.

## **Correct Answer:** D. Staphylococcus aureus