Answer: C

One of the most common benign skin tumors, seborrheic keratoses (SKs) are the result of epithelial proliferation associated with increasing age and chronic exposure to ultraviolet (UV) light. The lesions can vary in size and color but classically manifest as pigmented plaques or papules with a waxy, verrucous surface and a characteristic “stuck on” appearance.1,2 Because SKs are benign lesions, they do not require therapeutic removal; however, features such as dark coloration and irregular borders may prompt a biopsy to exclude life-threatening diagnoses such as melanoma.2-4

The prevalence of SKs increases with increasing age, with lesions occurring in most adults aged >50 years.4 The number of individual lesions also increases with age, usually appearing in large numbers in older individuals.Despite occurring more frequently in the elderly, SKs have also been identified in younger patients, and the condition affects both men and women equally.2 SKs are most prevalent among white individuals with fair skin, while their variant, dermatosis papulosa nigra, is more common among those of African or Asian descent with darker skin tones.

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Although the etiology of SKs remains unknown, many lesions reveal mutations in the FGFR3 gene, which regulates cell proliferation, angiogenesis, and wound repair.5 Mutated FGFR3 typically is associated with congenital skeletal dysplasias, but the gene also is expressed in the keratinocytes of wounded skin.4 Benign skin tumors, particularly SKs, have been shown to express increased numbers of FGFR3 mutations, which may be the result of the cumulative effects of exposure to UV light with increasing age.5 In addition to age and chronic exposure to UV radiation, family history may be a risk factor for SKs.1,5 Due to the classic verrucous appearance of SKs, human papillomavirus (HPV) has also been suggested as a potential etiology; however, recent studies have not yet confirmed viral involvement.1,24

The diagnosis of SK is usually based on distinct clinical presentations. Lesions present as well-demarcated, round or oval plaques that vary in size and color, ranging from flesh-tone to brown and occasionally black, often with a rough and oily surface. Although the majority of SKs are found on the face and trunk, they can be found almost anywhere on the body, except the palms, soles, and mucous membranes.1,2 Dermoscopy can be useful when diagnosis is uncertain. The most widely recognized diagnostic criteria for SKs demonstrated by dermoscopy are pseudofollicular openings resembling comedones as well as keratin-filled pits that form invaginations on the surface of lesions, known as horn pseudocysts.2 Additional features that may improve the accuracy of diagnosis and decrease the need for biopsy include a “moth-eaten border” and clusters of hairpin blood vessels.6 If the diagnosis remains unclear after dermoscopy, biopsy should be performed to definitively differentiate SK from a malignancy. Lesions can be further classified into subtypes based on the wide variety of histologic features, the most common of which include acanthosis, papillomatosis, and hyperkeratosis.2

Although SKs are benign neoplasms of epidermal cells, they may resemble malignant melanoma, which should be included in the differential diagnosis. Melanoma typically appears as an isolated lesion, whereas multiple SKs are commonly found at initial presentation, especially with increasing age.1,2 Malignant lesions evolve in appearance and size as they progress; in contrast, SKs have a greasy surface, appear stuck on the skin, and rarely display changes over time.3 The histologic appearance of SKs reveals a noninvasive, benign process, whereas melanoma is characterized by marked cellular atypia and infiltration.3 Other lesions with a similar clinical presentation as SKs of pigmented or rough plaques include actinic keratoses, melanocytic nevi, lentigo, or manifestations of HPV infection, such as verruca vulgaris or condyloma acuminatum.2 A rapid increase in size and number of symptomatic, pruritic SKs, a manifestation known as the Leser-Trélat sign, may indicate an underlying malignancy, most commonly adenocarcinoma of gastrointestinal origin, and requires an extensive diagnostic investigation.7

Because SKs are benign lesions, no therapy is necessary; however, they are removed frequently due to cosmetic concerns or if significant irritation occurs.1 Features of SKs such as bleeding, ulceration, and inflammation require further evaluation to rule out malignancy. Lesions are removed with a variety of operative techniques, including cryotherapy, curettage, and shave excision.2 Cryotherapy is a convenient and commonly used treatment method, although this technique is the most likely to leave a residual lesion.1 In contrast, shave excisions and curettage exchange more complete removal for increased risk for scar formation.

The patient in the case presented was reassured that the lesions were benign SKs. He was told that no intervention was necessary, but the patient requested removal of the lesion on his back. The lesion was initially treated with cryotherapy, but the patient opted for a shave excision after recurrence. The biopsy revealed horn pseudocysts consistent with an initial diagnosis of SK.

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