The correct answer is terra firma-forme dermatosis (TFFD; choice “d”), also known as “Duncan’s dirty dermatosis.” This relatively common condition usually affects adolescents and is one of the very few causes of hyperpigmentation that can be removed specifically with alcohol.
Dirty skin (choice “a”) is certainly seen, especially in this age-group, but the dirt is easily removed with soap and water.
Acanthosis nigricans (choice “b”) is often mistaken for dirty skin, but it cannot be removed by any nondestructive modality. Moreover, the most common form of acanthosis nigricans presents with a velvety, faintly raised brownish discoloration that usually affects the circumferential neck, axillae, and often, other intertriginous areas.
Reticulated and confluent papillomatosis (choice “c”) is a rare condition seen on the chest, back, and occasionally the face. It involves a slightly papular reticular (a netlike effect) patch, often in a triangular shape. Alcohol has no effect on it.
TFFD is surprisingly common, once its existence is recognized. Its etiology is, as one might expect, unknown, but it has been described in the literature (see “Suggested Reading” for examples) and is also defined by predictable histologic features seen on biopsy.
Besides the obvious implications, TFFD is probably most important as an imitator of acanthosis nigricans, which is often seen in overweight adolescents on their way to becoming diabetic. Unlike TFFD, acanthosis nigricans (type III, the most common form) has a multitude of potentially serious implications, although it is most often benign. Besides its well-known potential connection with diabetes, acanthosis nigricans can be seen in a myriad of insulin-resistant states and a bewildering variety of endocrinopathies.
Unnecessary treatments and/or workups can be avoided by being aware of the existence of this common condition, which is easily diagnosed (and treated!) by wiping with alcohol—effectively ruling out the other items in the differential.
Duncan WC, Tschen JA, Knox JM. Terra firma-forme dermatosis. Arch Dermatol. 1987; 123(5):567-569.
Pavlovic MD, Dragos V, Potocnik M, Adamic M. Terra firma-forme dermatosis in a child. Acta Dermatovenerol Alp Panonica Adriat. 2008;17(1):41-42.