The correct answer is idiopathic guttate hypomelanosis (choice “d”), a common condition primarily seen on sun-damaged patients, but often mistaken for tinea versicolor (choice “b”).
Vitiligo (choice “a”) can present with “confetti” lesions, but would more likely lead to complete loss of pigment in most of the lesions, not the partial loss seen in this patient. Biopsy is indicated in questionable cases.
Tinea versicolor, equally common, is caused by a commensal yeast called Malassezia furfur that feeds on sebum, which is why it favors the oily parts of the body (back and chest, primarily). It is almost never seen on the legs, which have the fewest oil glands on the body. In addition, tinea versicolor is, by its nature, an epidermal process, leading to the formation of a fine KOH-positive scale.
Since lupus (choice “c”) is a form of vasculitis, the associated inflammation can lead to pigment loss, especially in darker-skinned patients. Without a more likely explanation for these lesions, a biopsy might well have been indicated.
Biopsy might have suggested any number of other diseases that can also present with hypomelanosis, such as cutaneous T-cell lymphoma or sarcoid. But idiopathic guttate hypomelanosis is far more common, and the patient’s age, gender, and history of chronic UV damage all lend themselves perfectly to this diagnosis.
Idiopathic guttate hypomelanosis, for unknown reasons, tends to appear in women at earlier ages than in men (usually about a decade younger). For either gender, treatment is problematic once the lesions have fully developed. Early in the process, the obvious remedy is better sun protection. Lasers, retinoids, liquid nitrogen, and anti-inflammatory creams have all been tried with little success.