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You seem to be having a fungal infection
versicolor. It is caused by an yeast called malassezia
. This yeast is a normal resident of the skin, therefore there is a chance of recurrence. Usually with sweating you may feel mild itching. Neck and upper back is a common location. You may use an anti fungal cream like miconazole
twice a day for two weeks. Better to avoid oil or moisturizing creams as this yeast is lipophilic and loves oil.
Hope this helped