The correct answer is granuloma annulare (GA; choice “a”).
One of the most difficult concepts to grasp in dermatologic diagnosis is that almost all lesions and conditions—even the most common—have a broad range of morphologic presentations. These will often differ from the textbook photos. In this digital age, a simple Internet search will provide many results showing a diverse morphologic spectrum for many diseases, conditions, and lesions.
GA is a good example of how the presentation can vary. It has raised rolled margins and delled (gently concave) centers. The brownish red color is typical, but this patient showed a deeper red than most cases of GA. Occasionally, the red color is even deeper, with large patches of darkened skin and no palpable component.
Unfortunately, the misdiagnosis of fungal infection in this patient is typical. But dermatophytosis (otherwise known as “ringworm”)—the most common fungal skin infection—involves the epidermis. This means the patient would have scaly skin, probably with a well-defined margin—factors missing in this case. In addition, this patient reported no contact with sources of infection: animals, children, or immunosuppressive agents.
Ultimately, the most basic information that this patient's past providers neglected was a differential diagnosis. By establishing a differential, there was a clear decision to biopsy, which not only revealed the correct diagnosis but effectively ruled out the other options.
In defense of the patient’s other providers, I must admit that as a young primary care provider, I made this same mistake for exactly the same reasons.
Most cases of GA are mild and self-limited, requiring no treatment. This is fortunate because no effective treatment exists. Topical steroids and cryotherapy will lighten the lesion, but GA nearly always resolves on its own.