Pityriasis is one of any of a number of skin diseases that have in common lesions that resemble dandruff-like scales without obvious signs of inflammation.Types of pityriasis :Pityriasis alba: This is characterized by hypopigmented, round to oval, scaling patches on the face, upper arms, neck, or shoulders. The patches vary in size, usually being a few centimeters in diameter. The color is white or light pink. The scales are fine and adherent.

Usually, the patches are sharply demarcated; the edges may be erythematous and slightly elevated. As a rule, pityriasis is asymptomatic. However, there may be mild pruritis. The disease occurs chiefly in children and teenagers.

The cause is unknown. Excessively dry skin following exposure to strong sunlight appears to be contributory. Efforts to find an infectious agent – either bacterial, viral, or fungal – have been unsuccessful.Pityriasis rubra pilaris : This is a chronic skin disease characterized by small follicular papules, disseminated yellowish pink scaling patches, and often, solid confluent palmoplantar hyperkeratosis.

The papules are the most important diagnostic feature, being more or less acuminate, reddish brown, about pinhead size, and topped by a central horny plug. In the horn center a hair, or part of one, is usually embedded. The disease generally manifests itself first by scaliness and erythema of the scalp.

The eruption is limited in the beginning, having a predilection for the sides of the neck and trunk and the extensor surfaces of the extremities. Then, as new lesions occur, extensive areas are converted into sharply marginated patches of various sizes, which look like exaggerated goose-flesh and feel like a nutmeg grater. The involvement is generally symmetric and diffuse with, however, characteristic small islands of normal skin within the affected areas.