Quiz 1: What is your diagnosis?

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Diagnosis: Quiz 1

Quiz 1

Answer:  Clark's nevus

Criteria for diagnosis clinically: A tiny, round, dark brown macule with a subtle narrow tan rim positioned on volar skin is a Clark's nevus.

Differential diagnosis clinically: There is none.

Criteria for diagnosis histopathologically: Small discrete nests of markedly pigmented, strikingly dendritic melanocytes situated at the base of four or five contiguous rete ridges that themselves are darkened prominently by melanin, that pigment being present, too, in columns in the stratum corneum, are typical of a superficial congenital nevus of one type.

Differential diagnosis histopathologically: There is none.

Clinicopathologic correlation: The lesion is flat because the nests of melanocytes at the dermoepidermal junction are few and petite. It is dark brown throughout most of the macule because so much melanin is housed not only in nests of melanocytes themselves but in the viable and nonviable epidermis above them. It is tan at the periphery because fewer melanocytes and less melanin are present in the epidermis there. The surface is smooth because the stratum corneum, apart from being pigmented in loci linear, is normal.

Options for therapy predicated on knowledge of histopathologic findings: The lesion has been removed completely in this punched-out biopsy specimen. Because it surely is benign clinically, in theory it need not have been biopsied.

1) Clark's nevus presents itself clinically on volar skin as a small brown macule that histopathologically is either entirely junctional, as is the case here, or is compound with a dermal component of small nests of melanocytes in the uppermost part of the dermis. The term "acral nevus," unmodified, is inadequate and misleading because very different kinds of melanocytic nevi appear on an acral part alone, including volar skin, such as Spitz's nevus or on an acra and elsewhere concurrently, such as giant hairy nevus.

2) A few melanocytes seem to be lodged in a locus in the lower half of the spinous zone, that being a finding expected in junctional and compound nevi of all types situated on a palm and especially, on a sole. In no small number of different types of melanocytic nevi, including Spitz's and Zitelli's, melanocytes of the nevus may be situated far above the dermoepidermal junction, even in the cornified layer. In fact, the brown specks encircled partially by a tiny cleft in the stratum corneum of the nevus pictured here are melanocytes, they having retracted artefactually from adjacent pigmented corneocytes.

3) A discrete column of melanin is present in the stratum corneum immediately above nests of melanocytes positioned at the dermoepidermal junction. Such discrete columns are evidence of benignancy; they are not encountered in melanoma. Numerous melanophages are present in the vicinity of the nests pigmented prominently.

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