ANSWER
The correct answer is a form of psoriasis (choice “a”)—in this case, pustular psoriasis, which is seen mostly on hands and feet.
As in many such cases, biopsy was necessary; it successfully ruled out several items in the differential, in particular contact dermatitis (choice “b”). Ringworm and athlete’s foot (choice “c” and “d,” respectively) are archaic lay terms for fungal infection, which was not only ruled out by its absence in the biopsy specimen, but was unlikely given the lack of response to multiple antifungal medications.
DISCUSSION
Palmoplantar pustulosis is the term most often used to describe a fairly common form of psoriasis typified by this case. Many patients are genetically predisposed to psoriasis, but they may require a trigger to set it off, such as strep infection or occasionally, medication. Notable among the latter are the b-blockers and lithium. Stress is often involved as well.
The bilateral symmetrical involvement of both insteps is highly suggestive of this diagnosis, which often also affects either peripheral or central palms. A secondary form of neurodermatitis (itch–scratch–itch cycle) can follow, complicating the picture and perpetuating the problem.
Biopsy was deemed necessary to clarify the diagnosis and gain confidence in the therapeutic process, which can be difficult with this problem.
The patient was urged to consult her psychiatrist about finding a substitute for her lithium. At the time of this description, she is currently applying clobetasol cream under occlusion for three weeks, at the conclusion of which she will be re-evaluated. Many such patients go on to require the use of other medications, such as methotrexate or acetretin.