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The first thing that comes to my mind is a disease called atrophic rhinitis. Here, the mucous membrane and structures inside the nose shrink and the nasal cavity is filled with crusts, which get infected and smell. In this condition, the patient has decreased sense of smell, but others can feel bad smell coming out from the patient's nose.
A diagnostic nasal endoscopy is sufficient to diagnose the condition. It can specify whether it is atrophic rhinitis or something else.
If it is at all atrophic rhinitis, the treatment is frequent saline nasal washes and mechanical removal of the crusts by an ENT specialist. Rarely surgery is needed.
For more information consult an ENT otolaryngologist online --> https://www.icliniq.com/ask-a-doctor-online/ENT-Otolaryngologist