Quiz 62: What is your diagnosis?

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Diagnosis: Quiz 62

Quiz 62

Answer: Unna's nevus

Criteria for diagnosis clinically: A slightly rust-colored, smooth-surfaced, gently mamillate papule is an Unna's nevus.

Differential diagnosis clinically: This is none other than a fibroepithelial papilloma, but the anatomic site on which this lesion reposes favors strongly a nevus.

Criteria for diagnosis histopathologically: A dome-shaped lesion with a gently papillate surface within the dermis of which are monomorphic abnormal melanocytes disposed as solitary units, but also arranged in nests and in aggregations of different sizes and shapes, those in an "umbrella" in the uppermost part of the reticular dermis being pigmented markedly by melanin, is an Unna's nevus.

Differential diagnosis histopathologically: There is none. 

Clinicopathologic correlation: The papule consists of an infiltrate of abnormal melanocytes of the nevus, the rust color is a consequence of a combination of melanin in the cytoplasm of the abnormal melanocytes residing in the uppermost part of the reticular dermis and of erythrocytes in venules dilated widely in vivo in the reticular dermis, and the surface is smooth because the cornified layer is normal.

Options for therapy predicated on knowledge of histopathologic findings: Nothing more need be done because this lesion is benign.

1) The lesion fulfills criteria, clinical and histopathologic, for one type of melanocytic nevus, namely, Unna's. Although it fulfills criteria histopathologic for a congenital nevus, an Unna's nevus usually makes its appearance after birth, usually in childhood. The same is true for a Miescher's nevus. In short, not all congenital nevi actually are present at birth.

2) In the upper part of the reticular dermis, in addition to the usual nests, cords, and strands of abnormal melanocytes of the nevus, there are structures that resemble tiny nerve fascicles, they representing neural differentiation of the nevus. Those distinct units, known also as "neural tubes" and lame foliacée (French for a "thin plate [lame] laminated as if by layers of leaves [foliacée]") are encountered often in some types of congenital nevi that affect the reticular dermis, such as Unna's, but never, ever, are they met with in an acquired nevus that involves only the papillary dermis, such as Clark's.

3) In the epidermis on the left side of this lesion, as it is visualized at scanning power magnification, there are signs, indubitable, of infection by human papillomavirus. In two distinct loci of what appear to be infundibula of epidermis, keratocytes display larger, paler nuclei and abundant lightly basophilic cytoplasm. Those cells are arrayed both as solitary units and in clusters. The ones in the granular zone house within their cytoplasm prominent keratohyaline granules. Not only can those changes be identified as being the result of effects on keratocytes of infection by papillomavirus, but the attributes cytopathologic are those of that particular type of papillomavirus which is responsible for epidermodysplasia verruciformis (one expression of verruca plana). Their presence here, however, in no way implies that this lesion is en route to development of squamous-cell carcinoma, as is the case episodically for epidermodysplasia verriciformis.

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