: A localized overgrowth of the upper layer of skin. Common forms of keratosis include aging (senile keratosis) and sun exposure (actinic keratosis).
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Any lesion on the epidermis marked by the presence of circumscribed overgrowths of the horny layer. [kerato- + G. -osis, condition]
- actinic k. a premalignant warty lesion occurring on the sun-exposed skin of the face or hands in aged light-skinned persons; hyperkeratosis may form a cutaneous horn, and squamous cell carcinoma of low-grade malignancy may develop in a small proportion of untreated patients. SYN: senile keratoderma, senile keratoma, senile k., k. senilis, senile wart, solar k., verruca plana senilis, verruca senilis.
- arsenical k. multiple punctate keratoses, most commonly of the palms and soles but also of the fingers and proximal portions of the extremities, resulting from long-term arsenic ingestion; they resemble Bowen disease microscopically and may become squamous cell or basal cell carcinoma.
- k. blennorrhagica pustules and crusts associated with Reiter syndrome. SYN: keratoderma blennorrhagica.
- k. follicularis [MIM*124200] an autosomal dominant disorder characterized by eruption, beginning usually in late childhood, in which keratotic papules originating from both follicles and interfollicular epidermis of the trunk, face, scalp, and axillae become crusted and verrucous; the papules are often intensely pruritic. Microscopically, dyskeratotic cells termed corps ronds are seen in the epidermis. Longitudinal nail bands are frequent. SYN: Darier disease.
- inverted follicular k. a solitary benign epithelial tumor of infundibular hair follicle origin occurring on the face, consisting of a lobulated epidermal downgrowth of keratinizing squamous cells with a pattern of eddies or whorls.
- k. labialis thickening of stratum corneum on the lips.
- lichenoid k. a solitary benign papule or plaque, with microscopic features resembling lichen planus, occurring on sun-exposed or unexposed skin. SYN: lichen planus-like k..
- lichen planus-like k. SYN: lichenoid k..
- k. obturans an accretion of epithelia in the external auditory canal. SYN: laminated epithelial plug.
- k. palmaris et plantaris SYN: palmoplantar keratoderma.
- k. pilaris a common benign eruption consisting of scaly papules of the follicles; primarily affects the extensor surfaces of the arms and thighs.
- k. pilaris atrophicans faciei erythema and horny plugs of outer portions of the eyebrows with destruction of follicles; onset in early infancy.
- k. punctata SYN: punctate keratoderma.
- seborrheic k., k. seborrheica superficial, benign, verrucous, often pigmented, greasy lesions consisting of proliferating epidermal cells, resembling basal cells, enclosing horn cysts; they usually occur after the third decade. SYN: basal cell papilloma, seborrheic verruca.
- senile k., k. senilis SYN: actinic k..
- solar k. SYN: actinic k..
- tar k. warty lesions of the face and hands resulting from repeated, prolonged exposure to tar and pitch; also occurs as keratoacanthoma-like lesions that can become malignant, particularly on the scrotum.

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ker·a·to·sis .ker-ə-'tō-səs n, pl -to·ses -.sēz
1) a disease of the skin marked by overgrowth of horny tissue
2) an area of the skin affected with keratosis
ker·a·tot·ic -'tät-ik adj

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a horny overgrowth of the skin. Actinic (or solar) keratoses are red spots with a scaly surface, found in older fair-skinned people who have been chronically overexposed to the sun. They may occasionally become malignant. Seborrhoeic keratoses (or basal-cell papillomas), less correctly known as seborrhoeic warts, never become malignant. They are superficial yellowish spots, occurring especially on the trunk in middle age, that slowly darken and become warty over the years.

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ker·a·to·sis (ker″ə-toґsis) pl. keratoґses [kerato- + -osis] any horny growth, such as a wart or callus; the most common types are actinic keratosis and seborrheic keratosis.

Medical dictionary. 2011.

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