**Core Concept**
Malignant otitis externa is a severe infection of the external auditory canal, typically seen in diabetic or immunocompromised patients. It is characterized by its aggressive nature, involving the destruction of cartilage and bone in the external auditory canal.

**Why the Correct Answer is Right**
Malignant otitis externa is most commonly caused by Pseudomonas aeruginosa, a gram-negative bacterium that thrives in moist environments. This bacterium produces several virulence factors, including exotoxin A and elastase, which contribute to its ability to invade and destroy tissue. The infection often begins as a minor scratch or trauma to the external auditory canal, which becomes secondarily infected with Pseudomonas aeruginosa.

**Why Each Wrong Option is Incorrect**
* **Option A:** Staphylococcus aureus is a common cause of skin infections, but it is not the primary cause of malignant otitis externa.
* **Option B:** Escherichia coli is a gram-negative bacterium that can cause urinary tract infections, but it is not typically associated with malignant otitis externa.
* **Option C:** Streptococcus pneumoniae is a gram-positive bacterium that can cause otitis media, but it is not the primary cause of malignant otitis externa.

**Clinical Pearl / High-Yield Fact**
Malignant otitis externa is a medical emergency that requires prompt treatment with antibiotics, typically a combination of fluoroquinolones and cephalosporins. Early recognition and treatment can prevent serious complications, including facial paralysis and intracranial extension.

**Correct Answer: D. Pseudomonas aeruginosa**