**Core Concept**
The question is testing the understanding of the pathophysiology of external ear infections, specifically in a high-risk population such as diabetic elderly individuals. The underlying principle involves the compromised immune status and altered microbiome in this group, making them more susceptible to opportunistic infections.

**Why the Correct Answer is Right**
In diabetic elderly individuals, the virulent external ear infection is often caused by **Pseudomonas aeruginosa**. This bacterium takes advantage of the compromised immune system and the moist environment of the ear canal to cause a severe infection. The altered pH and temperature of the ear canal also create an ideal environment for the growth of Pseudomonas aeruginosa. The infection can progress rapidly, leading to severe pain, discharge, and potentially life-threatening complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Staphylococcus aureus is more commonly associated with skin and soft tissue infections, rather than external ear infections in diabetic elderly individuals.
**Option B:** This option is incorrect because Escherichia coli is more commonly associated with urinary tract infections and gastrointestinal infections, rather than external ear infections.
**Option C:** This option is incorrect because Aspergillus species are more commonly associated with fungal infections, such as aspergillosis, rather than bacterial external ear infections.

**Clinical Pearl / High-Yield Fact**
In diabetic elderly individuals, it is essential to suspect Pseudomonas aeruginosa as the causative agent of external ear infections and to initiate prompt and aggressive treatment to prevent complications.

**Correct Answer:** C. Pseudomonas aeruginosa.