Mystery of the rash

12 yr old otherwise healthy young man presents to his primary care physician office with a rash on the right calf for about a week.

He was admitted to the hospital for cellulitis. The rash is primarily on the posterolateral aspect of the proximal third of the right calf. The site is pruritic. There are similar rashes on both forearms. There was no history of trauma or travel. He does is a catcher on his baseball team but has not used any new equipment. He denies any fever or chills.
On examination he is well built nontoxic with stable vitals. His labs were unremarkable.
This is a picture of the calf rash:

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Notice the rectangular appearance and right angled corners. The surface has blisters of different sizes an age. The base of a popped blister is healthy skin. This is different from cellulitis where the dermal layer is inflamed. No pus is seen.
This is a photo of the arms.

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Notice the symmetry of the rash on both arms and the rectangular shape.

Diagnosis
Contact dermatitis
Why: multiple sites with sharp margins and geometric shape in a patient without history of trauma or systemic signs of febrile illness. The blisters are superficial with healthy bases. The dermatitis is from the outside in. Cellulitis would be from the inside out.
Upon taking further history it was determined that he has been sitting on a new chair with wooden arm rests and a wood plank where the calf would rest. He wears shorts and a T-shirt around the house. He has lately been using this chair for playing video games.
Contact dermatitis can occur with wood such as pine and yew.
He was discharged with local care and advised to avoid the chair.

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