A 70-year-old patient with long-standing type 2 diabetes mellitus presents with complaints of pain in the left ear with purulent drainage. On physical exam, the patient is afebrile. The pinna of the left ear is tender, and the external auditory canal is swollen and edematous. The peripheral white blood cell count is normal. The organism most likely to grow from the purulent drainage is

**Core Concept**
Diabetes mellitus increases the risk of skin and soft tissue infections, particularly those caused by bacteria that thrive in warm, moist environments, such as the external auditory canal. The patient's symptoms and physical examination findings are consistent with an external ear infection, also known as otitis externa.

**Why the Correct Answer is Right**
The most common cause of otitis externa in diabetic patients is Pseudomonas aeruginosa, a gram-negative bacterium that is highly opportunistic and can infect damaged or compromised tissue. The organism's ability to produce pyocyanin and other virulence factors allows it to thrive in the warm, moist environment of the external auditory canal. The patient's long-standing diabetes mellitus and the presence of a swollen, edematous external auditory canal create an ideal environment for Pseudomonas aeruginosa to cause infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus aureus is a common cause of skin and soft tissue infections, but it is more typically associated with abscesses or cellulitis rather than otitis externa.
**Option B:** Escherichia coli is a gram-negative bacterium that can cause urinary tract infections and other types of infections, but it is not the most likely cause of otitis externa in diabetic patients.
**Option C:** Candida albicans is a fungus that can cause skin and mucous membrane infections, particularly in immunocompromised patients, but it is not typically associated with otitis externa.

**Clinical Pearl / High-Yield Fact**
In diabetic patients, otitis externa is often a sign of more widespread infection or sepsis, and prompt treatment with antibiotics and wound care is essential to prevent further complications.

**Correct Answer:** C.

✓ Correct Answer: A. Pseudomonas aeruginosa