**Question:** Organism causing otitis externa includes all except

A. Staphylococcus epidermidis
B. Pseudomonas aeruginosa
C. Candida albicans
D. Aspergillus fumigatus

**Correct Answer:** D. Aspergillus fumigatus

**Core Concept:**
Otitis externa, also known as swimmer's ear, is an inflammation of the outer ear canal caused by various pathogens. The correct answer, Aspergillus fumigatus, is an ascomycete fungus commonly associated with invasive infections in immunocompromised individuals. Most cases of otitis externa are caused by bacteria, particularly gram-negative rods like Pseudomonas aeruginosa and gram-positive cocci like Staphylococcus epidermidis and Streptococcus pyogenes. Additionally, Candida species like Candida albicans can also cause otitis externa in immunocompromised patients.

**Why the Correct Answer is Right:**
Aspergillus fumigatus is a saprophytic fungus primarily causing invasive infections in immunocompromised patients. It is not commonly associated with otitis externa, as the condition is typically caused by bacteria or yeasts like Candida albicans in the context of a healthy immune system.

**Why Each Wrong Option is Incorrect:**

A. Staphylococcus epidermidis: This is a commensal bacterium found on the skin and mucous membranes. Although it can cause opportunistic infections, otitis externa is not one of its primary manifestations.

B. Pseudomonas aeruginosa: This is a common gram-negative bacterium found in water and soil. It is often associated with otitis externa, especially in warm and humid environments, making it a wrong option in this context.

C. Candida albicans: This is a common yeast found on the skin and mucous membranes. While it can cause opportunistic infections, otitis externa is not one of its primary manifestations.

**Clinical Pearls:**

1. Aspergillus fumigatus is a case of invasive infection, not otitis externa.
2. Bacteria like Staphylococcus epidermidis, Pseudomonas aeruginosa, and Candida albicans are common pathogens causing otitis externa, particularly in susceptible individuals.
3. In healthy individuals, most cases of otitis externa are caused by bacteria like Pseudomonas aeruginosa, Staphylococcus epidermidis, and Streptococcus pyogenes, while Candida species like Candida albicans can be involved in immunocompromised patients.