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Sun burn can be excluded as it happens once a while.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided.
It will be great if you mention is there in history of injury or any history of allergies like have you come across something that you usually won't.
I advice you to recall is there any thing your wife has done when she had rash like using particular perfume, cloth or soap or jogging she might have come in contact with some plant.
your symptoms are suggestive of contact dermatitis, a itchy rash caused by things your skin touches, such as: cosmetics, soaps, and detergents, dyes and other chemicals in clothing, rubber gloves, shampoos, antibiotics such as neomycin rubbed on the surface of the skin or any plants.
Treatment includes washing with lots of water to remove any traces of the irritant that may remain on the skin. You should avoid further exposure to known irritants or allergens. In some cases, the best treatment is to do nothing to the area.
Emollients or moisturizers help keep the skin moist, and also help skin repair itself. They protect the skin from becoming inflamed again. They are a key part of preventing and treating contact dermatitis.
Corticosteroid skin creams or ointments may reduce inflammation. Overuse, even of low-strength over-the-counter products, may cause a skin condition should be used with caution.
I advice you to follow this above mentioned measures still if it persists or if it continues to occur, consult the dermatologist for examination and also to find out the cause. As it appears once a while it might also be due to insect bite.
Hope I have provided the information you needed.
Wish you good health. Take care.

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