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40-Year-Old Female with Itchy Spot on Forearm

Can you diagnose this case?

David L. Kaplan, MD – 2015 Series Editor

Signs and Symptoms

This 40-year-old woman presented for evaluation of an itchy spot on her forearm that had suddenly turned black. She had been cleaning around the house and doing yard work the day before. She was otherwise healthy. She did not remember any exposure history or bites, and she has no previous history of a mole.

What explains this woman’s lesion?

Choose one to reveal diagnosis and discussion

Brown recluse spider bite
Poison ivy reaction
Melanoma
Seborrheic keratosis
Impetigo

Answer: Poison ivy reaction

See the full case at Consultant360

This woman’s lesion was the result of poison ivy contact dermatitis, with the urushiol oxidizing to a black color upon exposure to air. Brown recluse spider bites are painful, not itchy, with a more necrotic appearance. Melanoma or seborrheic keratosis would not appear overnight and would not be expected to be itchy. Impetigo would be itchy, but it typically has a more erosive appearance with serum crusting, unlike what is seen here.

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