**Core Concept**
Pseudomonas aeruginosa is a gram-negative bacterium known for its ability to produce a distinctive blue-green pigment called pyocyanin, as well as an ADP-ribosylation toxin called exotoxin A. This bacterium is notorious for causing opportunistic infections, particularly in immunocompromised individuals and those with chronic illnesses like diabetes.

**Why the Correct Answer is Right**
Pseudomonas aeruginosa is a common cause of external otitis, especially in individuals with compromised immune systems. The production of pyocyanin is a key virulence factor, allowing the bacterium to outcompete other microorganisms and establish infection. Exotoxin A is a potent inhibitor of protein synthesis, contributing to the bacterium's ability to cause tissue damage and sepsis. The combination of pyocyanin production and exotoxin A secretion makes Pseudomonas aeruginosa a prime suspect in cases of severe external otitis with bone and nerve damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus epidermidis is a coagulase-negative staphylococcus that is commonly found on human skin. While it can cause infections, particularly in immunocompromised individuals, it does not produce pyocyanin or exotoxin A.

**Option B:** Staphylococcus aureus is a gram-positive bacterium that can produce a range of virulence factors, including coagulase and enterotoxins. However, it does not produce pyocyanin or exotoxin A, making it an unlikely cause of this particular infection.

**Option D:** Enterococcus faecalis is a gram-positive bacterium that is commonly found in the human gut. While it can cause infections, particularly in urinary tract and endocarditis, it does not produce pyocyanin or exotoxin A.

**Clinical Pearl / High-Yield Fact**
Pseudomonas aeruginosa is a classic example of an opportunistic pathogen that thrives in environments with compromised immune function. Its ability to produce pyocyanin and exotoxin A makes it a formidable foe in the context of external otitis, and clinicians should be vigilant in diagnosing and treating this bacterium in patients with underlying medical conditions.

✓ Correct Answer: C. Pseudomonas aeruginosa