This 66-year-old woman has had a very itchy rash on her left foot for several months. She has tried applying a number of different OTC and prescription medications—including betamethasone dipropionate cream and econazole cream—without successful resolution of the problem.
She denies having any other skin problems, and there is no relevant family history. The patient is retired and lives alone with her cat. Medical history is remarkable for rheumatoid arthritis, for which she takes methotrexate.
The dorsum of her left foot is covered with a sharply demarcated, papulosquamous red rash. Interestingly, the interdigital areas are spared, as are the sole and the entire right foot. The patient’s elbows, knees, and scalp exhibit no significant changes. On further examination, a fine, slightly pink, powdery rash is noted on the sides of both feet (including the heels).
A KOH prep of the rim of the feet is positive for fungal elements, as is a similar microscopic examination of scrapings from the dorsum of the left foot. In fact, the fungal elements seen on the latter are so numerous and dense that they are initially difficult to see.
What is the diagnosis?