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
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
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