A 23-year-old woman presents to dermatology with an itchy rash she has had for several months. Although it manifested on her wrists and finger, the rash moves around and causes itching on her legs, trunk, and arms at various times. It has not affected her breasts or axillae.
The patient has been seen in primary care several times and received the usual topical steroids, antihistamines, and at least three courses of prednisone—none affording much relief.
She denies that anyone else in her household is itching. During her last visit to primary care, they treated her with topical permethrin cream, which was to be left on overnight then washed off. No relief was forthcoming.
Scattered areas of faint eczematoid rashes can be seen across her thighs and arms. There are two or three tiny excoriated papules on both volar wrists, but no intact vesicles are observed.
A closer inspection of her palms reveals one tiny linear vesicle on the mid right palm. Vigorous scraping with a #10 blade produces material, which is placed on a slide, covered, filled with potassium hydroxide 10%, and examined under a microscope.
What is the diagnosis?