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The diagnosis that you made yourself might be true because that is how Pityriasis Rosea
progress. When it is a herald patch stage , i e when only a single lesion is present even the best of the best doctors might mistake it for tinea, but once daughter patches start we can easily diagnose it. Moreover lack of response to ketoconazole
is against tinea. I would recommend you go back to the dermatologist
for a review for confirmation.
Pityriasis Rosea is usually self limiting. It will usually clear in 4 to 6 weeks. If it is not itchy or giving you any trouble you can just leave it alone. Usual cause is an infection, might be a mild viral fever
which occurs 1-2 weeks prior to onset of herald patch. Sometimes it might be due to certain medications.Diet, soap, stress
etc usually does not play a role but you may continue the good routine.
Hope this helped