The Itch That Won't Quit

A 68-year-old woman with a very itchy rash is referred to dermatology for evaluation.  She reports the itching to be constant—24 hours a day, seven days a week—but particularly severe at bedtime.

The rash has been totally unresponsive to numerous treatment attempts over the past year, including topical steroids (triamcinolone 0.1% cream bid), oral antibiotics (trimethoprim/sulfa), and oral steroids (prednisone).

The patient lives alone, apart from the occasional overnight visit from her grandchild.

EXAMINATION
The widespread rash spares only the patient’s legs below the knees. It is comprised of sparsely distributed excoriated foci, some surrounded by oval-to-round scales. The patient scratches the sites throughout the examination.

During a shave biopsy of one of the lesions on the patient’s arm, she mentions that occasionally lesions also manifest on her hands and fingers. Closer examination reveals a few unremarkable, scaly, 1- to 3-mm papules on her volar wrists. These are scraped with a #10 blade and placed on a slide, which is covered, filled with potassium hydroxide 10%, and examined under 10x magnification.

What is the diagnosis?