The most likely diagnosis is psoriasis (choice “d”), which can manifest with localized involvement (see discussion below).
Fungal infection (choice “a”) is unlikely, given the negative results of the KOH prep and total lack of response to treatment for that diagnosis.
Eczema (choice “b”) does not manifest as such thick, adherent scale. If any nail changes were involved, they would likely consist of transverse nail ridges.
A variant of squamous cell carcinoma (SCC; choice “c”)—caused by human papillomavirus (HPV), for example—can produce somewhat similar changes in the skin. But it would be unlikely to lead to nail changes, and it would not be intermittent, as this rash was in apparent response to OTC cream.
A punch biopsy is sometimes required to confirm the diagnosis of psoriasis, but this combination of skin and nail changes is quite suggestive of that entity. In this case, the confirmation was made by a different route: The rheumatologist judged that the patient’s arthritis was psoriatic in nature. The arthritis was treated with methotrexate, which soon cleared the skin disease without any help from dermatology.
This case serves to reinforce the possible mind-body role of stress in the genesis of this disease; however, at least 30% of the time, there is a positive family history. It also demonstrates the seemingly contradictory fact that the severity of the psoriasis doesn’t always correlate with the presence or absence of psoriatic arthritis.
Adding to the difficulty of making the diagnosis of psoriasis is the localized distribution of the scales, since it can usually be corroborated by finding lesions in their usual haunts—such as the extensor surfaces of arms, legs, on the trunk, or in the scalp. Cases such as this one force the provider to carefully consider the differential, which includes fungal infection. But the dermatophytes that cause ordinary fungal infections have to come from predictable sources (eg, children, pets, or farm animals), all missing from this patient’s history.
Had the rash been “fixed” (unchanging), the diagnosis of SCC would have to be considered more carefully. Superficial SCC is called Bowen’s disease, which, in cases such as this, is usually caused by HPV, not by the more typical overexposure to UV light. Though superficial, Bowen’s disease can become focally invasive and can even metastasize.
Had this patient not been treated with methotrexate, we would probably have used topical class 1 corticosteroid creams, which would have had a good chance to improve his skin but not his nail. The patient was also counseled regarding the role of stress and increased alcohol intake in the worsening of his disease. His prognosis for skin and joint disease is decent, though he will probably experience recurrences.