Pityriasis rosea

Pityriasis rosea is a common skin condition that causes a distinctive rash, primarily affecting the skin. It usually starts with a single large, scaly patch called a herald patch, followed by smaller patches that spread across the body. The rash often appears on the trunk and upper arms and legs, and it can cause mild itching. This condition mainly affects children and young adults and typically resolves on its own within several weeks to a few months. Although the exact cause is unknown, it is thought to be related to a viral infection. The rash is not contagious and does not cause serious health problems.

Pityriasis rosea is an acute, self-limited inflammatory skin disorder characterized by a distinctive herald patch followed by a generalized secondary eruption of oval, scaly plaques arranged along skin cleavage lines, producing a characteristic Christmas tree pattern. The etiology is believed to involve reactivation of human herpesvirus types HHV-6 and HHV-7, leading to a cell-mediated immune response in the skin. It primarily affects adolescents and young adults and is more common in the spring and fall. The condition is benign and typically resolves spontaneously within 6 to 8 weeks. Histopathology shows spongiosis, parakeratosis, and a superficial perivascular lymphocytic infiltrate. The major clinical significance lies in its differentiation from other papulosquamous disorders such as secondary syphilis and psoriasis.