**Core Concept**
Pseudomonas aeruginosa is a gram-negative, opportunistic pathogen that causes a wide range of infections, particularly in immunocompromised patients. It is known for its ability to form biofilms, which contribute to its resistance to antibiotics and host immune responses.

**Why the Correct Answer is Right**
Pseudomonas aeruginosa is a non-motile bacterium at room temperature but becomes motile at 37°C due to the expression of flagella. This temperature-dependent motility is a key factor in its pathogenesis. The bacterium produces various virulence factors, including exotoxin A, which inhibits protein synthesis, and elastase, which breaks down elastin, a key component of the extracellular matrix. Pseudomonas aeruginosa is also resistant to many antibiotics due to its ability to form biofilms and modify its cell wall.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Pseudomonas aeruginosa is indeed a gram-negative bacterium.
**Option B:** This option is incorrect because Pseudomonas aeruginosa is a known opportunistic pathogen that causes infections in immunocompromised patients.
**Option C:** This option is incorrect because Pseudomonas aeruginosa is indeed a non-motile bacterium at room temperature but becomes motile at 37°C.

**Clinical Pearl / High-Yield Fact**
Pseudomonas aeruginosa infections are often associated with cystic fibrosis, burn injuries, and immunocompromised states. Treatment of Pseudomonas aeruginosa infections requires combination therapy with antibiotics that target both the bacterium and the biofilm.

**Correct Answer: D.**