This 50-year-old African-American female developed a warty, hyperpigmented plaque in her axilla. It was submitted as a possible seborrheic keratosis. Granular parakeratosis is largely idiopathic but some authors feel it is due to external injury, such as antiperspirant use. Defects in profilaggrin-filaggrin processing have been suggested. The histology is characteristic in this case, showing retention of keratohyaline granules within the parakeratotic scale. The differential diagnosis would include other benign keratoses when the retention of granules is less prominent.
Histologic sections of skin show epidermal acanthosis with overlying mounds of hyperparakeratosis. The lesion has a flat bottom and is not infiltrative.