A 28-year-old male presents with acute severe otalgia and otorrhea for 2 days following swimming in a chlorinated pool. On examination, the external auditory canal shows erythema, edema, and purulent debris. Tragal tenderness is marked. An audiogram is obtained and shows the pattern marked **A** — a flat mild-to-moderate conductive hearing loss with intact bone-conduction thresholds across all frequencies. Which of the following is the most likely causative organism in this patient's condition?

Explanation

## Why Pseudomonas aeruginosa is right The clinical presentation — acute otalgia with tragal tenderness, otorrhea, canal edema and debris following water exposure, combined with the audiometric pattern marked **A** (flat mild-moderate conductive loss with normal bone conduction) — is pathognomonic for acute otitis externa (OE). Pseudomonas aeruginosa is the most common causative organism in bacterial OE, accounting for approximately 38% of cases. Water exposure disrupts the cerumen barrier and raises canal pH from acidic to neutral/alkaline, creating an ideal environment for Pseudomonas proliferation. The flat conductive loss results from canal occlusion by edema and debris, not ossicular pathology, which is the hallmark audiometric finding in uncomplicated OE (Dhingra, Ch 8; Cummings, Ch 138). ## Why each distractor is wrong - **Staphylococcus aureus**: While S. aureus is the second most common bacterial pathogen in OE (~8%), it is significantly less frequent than Pseudomonas. The clinical scenario with water exposure as a clear trigger strongly favors Pseudomonas, which thrives in moist environments. - **Aspergillus niger**: This fungal organism causes otomycosis, which typically presents with itching rather than severe otalgia, and shows characteristic "damp newspaper" or "blotting paper" appearance of black/white fungal hyphae on otoscopy. Fungal OE is more common in tropical climates and chronic cases, not acute post-swimming presentations. - **Streptococcus pneumoniae**: This organism is not associated with external auditory canal infection. It is a common cause of acute otitis media (middle-ear infection), which would present with tympanic membrane findings (bulging, erythema) and normal canal examination, not canal edema and debris. **High-Yield:** Acute OE with water exposure + tragal tenderness + flat conductive loss = Pseudomonas aeruginosa; treat with topical ciprofloxacin 0.3% drops after aural toilet. [cite: Dhingra Diseases of Ear Nose & Throat 7e Ch 8; Cummings Otolaryngology 7e Ch 138]