**Core Concept**
External otitis, also known as swimmer's ear, is a common inflammatory condition of the external auditory canal. It is often caused by bacterial or fungal infections, which can lead to discomfort, pain, and potentially serious complications if left untreated. The external auditory canal provides a warm, humid environment that is conducive to bacterial growth.

**Why the Correct Answer is Right**
Pseudomonas aeruginosa is a gram-negative bacterium that thrives in moist environments, making it a common cause of external otitis in adults. The bacteria can colonize the external auditory canal, particularly in individuals who have compromised skin integrity, such as those with eczema or psoriasis, or in individuals who have recently used earplugs or earphones. Pseudomonas aeruginosa produces virulence factors that enable it to adhere to the epithelial cells of the external auditory canal, leading to inflammation and infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Streptococcus viridans is a gram-positive bacterium that is commonly associated with endocarditis and other infections, particularly in individuals with underlying medical conditions. It is not typically a cause of external otitis.
**Option B:** Staphylococcus epidermidis is a gram-positive bacterium that is commonly found on the skin and mucous membranes. While it can cause infections, it is not typically a cause of external otitis.
**Option D:** E. coli is a gram-negative bacterium that is commonly associated with urinary tract infections and other infections, particularly in individuals with compromised immune systems. It is not typically a cause of external otitis.

**Clinical Pearl / High-Yield Fact**
In individuals with external otitis, it is essential to avoid using ear drops that contain hydrogen peroxide, as these can further dry out the affected area and exacerbate the infection.

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