**Core Concept**
Malignant otitis externa, also known as necrotizing otitis externa, is a severe infection of the external auditory canal that can lead to destruction of the cartilage and bone. This condition is often seen in immunocompromised patients, particularly those with diabetes mellitus.

**Why the Correct Answer is Right**
Malignant otitis externa is primarily caused by Pseudomonas aeruginosa, a gram-negative bacterium. This bacterium takes advantage of the compromised immune system and invades the external auditory canal, leading to tissue destruction and necrosis. The pathogenesis involves the production of elastase by Pseudomonas aeruginosa, which breaks down the cartilage and bone in the external auditory canal, causing the characteristic destruction seen in this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Aspergillus species can cause otomycosis, a fungal infection of the external auditory canal, but it is not the primary cause of malignant otitis externa.
**Option B:** Staphylococcus aureus can cause a variety of infections, including skin and soft tissue infections, but it is not the primary cause of malignant otitis externa.
**Option C:** Haemophilus influenzae can cause a variety of infections, including otitis media, but it is not the primary cause of malignant otitis externa.

**Clinical Pearl / High-Yield Fact**
Malignant otitis externa is often seen in patients with diabetes mellitus, and prompt treatment with antibiotics, including quinolones and aminoglycosides, is essential to prevent further tissue destruction and complications.

**Correct Answer: D. Pseudomonas aeruginosa**