About six months ago, an 8-year-old girl developed an asymptomatic rash near her ear. Her mother suspects it is psoriasis, which runs heavily in the family—but their primary care provider favors a fungal diagnosis. He prescribes a succession of topical and oral antifungal medications (including nystatin and terbinafine), which yield no discernable improvement. At this point, referral to dermatology is made.
The child’s mother denies any history of recent infections (eg, strep throat) on her daughter’s behalf. Furthermore, there are no reports of pain associated with the rash or elsewhere.
The rash, which is confined to the external right ear, is composed of uniformly smooth white scale on a faintly salmon base. The entire lesion measures about 3 cm at its widest point, and the margins are arciform and well-defined.
No such lesions are seen elsewhere, but tiny pits can be seen on one fingernail.
What is the diagnosis?