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Medical Definition of Hair cast
1. 1. A cast composed of parakeratotic scales attached to scalp hair but freely movable up and down the hair shaft; found in scaling dermatitis of the scalp, including dandruff, psoriasis, and seborrheic dermatitis. Synonym: pseudonit. 2. A small, nodular accretion of epithelial cells and keratinous debris resulting from failure of the internal root sheath to disintegrate; it appears for 3 to 7 mm along the hair shaft. (05 Mar 2000)