The correct answer is to perform a KOH prep (choice “d”), because it is the only choice that offers any chance of providing a definitive diagnosis—always preferable. It is true that if it worked, treatment with either trimethoprim/sulfa (choice “a”) or terbinafine (choice “b”) would suggest definitive diagnoses. However, if it failed, or only partially worked, at best you’d be right back where you started, or you might simply have confused the clinical picture.
Regarding betamethasone/clotrimazole (choice “c”), there is almost never a time when this combination should be used for anything. The antifungal ingredient is too weak to treat a serious fungal infection, and the steroid is far too strong for most applications. It’s “shotgun” therapy and to be avoided.
The KOH that was done in this case was wildly positive for fungal elements, establishing the etiology of the condition called Majocchi’s granuloma. This is a fungal folliculitis most commonly triggered by the application of topical steroids to skin already infected with dermatophytic fungal organisms. With local immune response thus dampened by the steroids, the organisms, which are normally confined to the epidermis, are allowed to migrate into the follicles and surrounding perifollicular tissues, where they become difficult to diagnose and treat.
The biopsy should have demonstrated this, but there are limitations to what can be seen on a 4-mm specimen. Luckily, there was scale to test, and a history of steroid application to corroborate the diagnosis. The strikingly linear borders of the condition were also helpful, since they were dictated by the limits of the hand application of the steroid ointment.
Treatment included daily terbinafine and topical econazole cream (bid) for two months. Needless to say, steroid use was stopped and the patient educated on its role in her condition.