Malignant otitis externa — DTM&H MCQ

Correct answer: B — Malignant (necrotising) otitis externa

The correct answer is B, malignant (necrotising) otitis externa. This diagnosis is defined by the combination of poorly controlled diabetes (a key risk factor via microvascular disease and impaired neutrophil function), granulation tissue at the bone-cartilage junction of the ear canal, Pseudomonas aeruginosa on culture, and CT evidence of temporal bone erosion, which together indicate skull base osteomyelitis arising from the external canal rather than simple soft-tissue infection. UK guidance explicitly flags granulation tissue or exposed bone in the canal, disproportionate pain, and diabetes or immunosuppression as the features that should trigger suspicion of this condition and prompt urgent ENT referral. The bony erosion on CT confirms progression beyond otitis externa into osteomyelitis, which is the defining pathological step separating this entity from its mimics. Why the other options are wrong: A. Chronic suppurative otitis media: This is a middle ear disease with a perforated tympanic membrane and chronic otorrhoea, not a canal-based granulation tissue lesion with temporal bone erosion; it does not typically cause skull base osteomyelitis. E. Otomycosis: Fungal otitis externa produces characteristic creamy white or black/white spore-laden debris rather than granulation tissue, and does not culture Pseudomonas or cause bony erosion on CT. D. Cholesteatoma: This arises from the middle ear or attic as a keratinising squamous lesion causing bone erosion through pressure and enzymatic activity, not from an infected external canal with a positive Pseudomonas culture. C. Otic tuberculosis: TB otitis media classically presents with painless otorrhoea, multiple tympanic perforations, and pale granulation tissue, and would not culture Pseudomonas aeruginosa or show this acute bony erosion pattern. Key point: Granulation tissue in the external canal plus Pseudomonas culture plus temporal bone erosion on CT in a diabetic patient is the classic triad confirming malignant (necrotising) otitis externa, mandating urgent ENT referral.

Reference: NICE CKS-based guidance via Patient.info (Otitis externa and painful, discharging ears, reviewed 2022, reflecting NICE CKS Otitis Externa, May 2024): suspect malignant otitis externa when there is granulation tissue at the bone-cartilage junction or exposed bone in the ear canal, with pain disproportionate to signs, especially in diabetic or immunosuppressed patients; urgent ENT referral is required. https://patient.info/doctor/otitis-externa-and-painful-discharging-ears