**Core Concept**
Diabetic patients are prone to various infections, particularly those caused by opportunistic pathogens. The presentation of necrosis of the external auditory meatus with foul-smelling discharge is suggestive of a specific type of otitis externa, often associated with a particular bacterium.

**Why the Correct Answer is Right**
The condition described is likely to be malignant otitis externa, a severe and potentially life-threatening infection. The presence of diabetes mellitus is a significant risk factor for this condition. Malignant otitis externa is typically caused by Pseudomonas aeruginosa, a gram-negative bacterium that thrives in moist environments. This organism produces virulence factors such as elastase and alkaline protease, which contribute to tissue damage and the characteristic foul-smelling discharge.

**Why Each Wrong Option is Incorrect**

* **Option A:** Staphylococcus aureus can cause various types of infections, but it is not the primary organism associated with malignant otitis externa.
* **Option B:** Escherichia coli is a common cause of urinary tract infections and other types of bacterial infections, but it is not typically linked to malignant otitis externa.
* **Option D:** Aspergillus species can cause fungal infections, particularly in immunocompromised individuals, but they are 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 broad-spectrum antibiotics, including antipseudomonal agents. Early recognition and treatment can prevent serious complications, such as facial nerve paralysis and intracranial extension.

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