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What is Parosmia and explain in detail?
Parosmia (from the Greek παρά pará and ὀσμή osmḗ "smell") is a dysfunctional smell detection characterized by the inability of the brain to correctly identify an odors "natural" smell. Instead, the natural odor is usually transformed into an unpleasant aroma, typically a "burned", "rotting", "fecal", or "chemical" smell. There can also be rare instances of a pleasant odor called euosmia. The condition was rare and little-researched until it became relatively more widespread since 2020 as a side effect of COVID-19.

Causes
There are numerous diseases with which parosmia is associated. In a case study, Frasnelli et al. examined five patients that endured parosmia or phantosmia, most as a result of upper respiratory tract infections (URTIs). It is hypothesized that URTIs can result in parosmia because of damage to olfactory receptor neurons (ORNs). The condition has been linked to coronavirus disease 2019 as a rare side effect. Common triggers in COVID-19 related parosmia include coffee, chocolate, meat, onion and toothpaste.  Exposure to harmful solvents has also been linked to parosmia, specifically by damaging ORNs.
Damage to these neurons could render them unable to correctly encode a signal representing a particular odor, which would send an erroneous signal to the odor processing center, the olfactory bulb. This, in turn, leads to the signal triggering a different smell than the stimulating odor, and thus the patient cannot sync the input and output odors. Damage to ORNs describes a peripheral defect in the pathway, but there are also instances where damage to the processing center in the brain can lead to distorted odors.
Different types of head trauma could also lead to dysfunctions that relate to what the afflicted brain area controls. In humans, the olfactory bulb is located on the inferior side of the brain. Physical damage to this area would alter how the area processes information in a variety of ways, but there are also diseases that can alter how this area works. If the part of the brain that interprets these input signals is damaged, then a distorted output is possible. This would also lead to parosmia. Temporal lobe epilepsy has led to cases of parosmia, but these were only temporary; the onset of parosmia was a seizure and it typically lasted a week or two after. Parosmia is sometimes associated with Parkinsons disease