**Core Concept**
Malignant otitis externa, also known as necrotizing otitis externa, is a severe infection of the external auditory canal and surrounding tissues. It is a type of osteomyelitis that affects the temporal bone.

**Why the Correct Answer is Right**
The causative organism of malignant otitis externa is typically Pseudomonas aeruginosa, a gram-negative bacterium. This bacterium is known to produce virulence factors such as elastase, alkaline protease, and exotoxin A, which contribute to its ability to cause invasive disease. In individuals with compromised immune systems, such as those with diabetes or undergoing chemotherapy, Pseudomonas aeruginosa can spread to the surrounding tissues, leading to necrotizing otitis externa.

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

**Clinical Pearl / High-Yield Fact**
Malignant otitis externa is a medical emergency that requires prompt treatment with antibiotics, including coverage for Pseudomonas aeruginosa. Early recognition and treatment can prevent serious complications, such as facial paralysis and intracranial extension.

**Correct Answer: D. Pseudomonas aeruginosa**