**Core Concept**
The question is testing the student's knowledge of the pathogenesis of external ear infections, particularly in a high-risk population like diabetic elderly individuals. The underlying concept is the increased susceptibility of diabetic patients to infections, especially those caused by opportunistic pathogens.

**Why the Correct Answer is Right**
In diabetic elderly individuals, the virulent external ear infection is mostly caused by Pseudomonas aeruginosa. This bacterium thrives in moist environments and takes advantage of the compromised immune system in diabetic patients. Pseudomonas aeruginosa produces enzymes like elastase and alkaline protease, which help it to invade and colonize the ear canal. The use of immunosuppressive agents, poor glycemic control, and pre-existing ear canal abnormalities contribute to the increased susceptibility to this infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** Staphylococcus aureus is a common cause of external ear infections, but it is more likely to cause acute, rather than virulent, infections in diabetic elderly individuals.
* **Option B:** Streptococcus pneumoniae can cause external ear infections, but it is not the primary pathogen in diabetic elderly individuals.
* **Option C:** Escherichia coli can cause external ear infections, but it is not typically associated with virulent infections in diabetic elderly individuals.

**Clinical Pearl / High-Yield Fact**
Pseudomonas aeruginosa is a common cause of external ear infections in diabetic patients, often presenting with symptoms like otorrhea, pain, and a foul odor. Treatment typically involves topical or systemic antibiotics, along with measures to improve glycemic control and dry the ear canal.

**Correct Answer:** C. Pseudomonas aeruginosa