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The described symptoms are suggestive of a fungal infection
referred to as tinea cruris
. Itching, soreness, stinging sensation and burning sensation in association with pigmented rash
is common symptom with this condition.
Ideally I would advice you to consult a dermatologist
who after careful examination of the affected areas can confirm the condition and treat it accordingly.
If an appointment with the dermatologist is far off, you may try topical antifungals without steroid
preparation such as flucanozole over the affected areas for 2-3 weeks. Keep the area clean and dry and maintain high standards of personal hygiene.
Hope this helps.