Prurigo nodularis (PN) is a skin disease that causes hard, itchy lumps (nodules) to form on the skin. The itching (pruritus) can be intense, causing people to scratch themselves to the point of bleeding or pain. Scratching can cause more skin lesions to appear. In some cases, people with PN have a history of other diseases including eczema (atopic dermatitis), lymphoma, HIV infection, severe anemia, or kidney disease. The exact cause of PN is unknown. Although scratching is known to cause more nodules to appear, it is unclear what causes the itching to develop in the first place. Diagnosis of the disease is based on observing signs such as extremely itchy skin with the formation of nodules. In some cases, a skin biopsy is used to confirm the diagnosis.