Atopic Dermatitis (Eczema): This is the most common cause of a papular, itchy rash, especially in children but also in adults. It's characterized by dry, itchy skin that can become red and inflamed, often appearing in the folds of the elbows or behind the knees.

Contact Dermatitis: An allergic reaction or irritation caused by substances like soaps, detergents, or metals, leading to itchy, red, and sometimes blistered skin.
Urticaria (Hives): Characterized by itchy, raised welts that can appear anywhere on the body, often due to allergic reactions, infections, or autoimmunity.
Papular Urticaria: A type of skin reaction often caused by insect bites, leading to itchy papules.
Flea Bites or Scabies: Infestations that cause intense itching and papular rashes, often in specific patterns or locations.

Bullous Pemphigoid: An autoimmune disease causing large, itchy blisters, which can be life-threatening if not treated.
Drug Rash (including Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis): Severe skin reactions to medications that can be life-threatening, presenting with itchy rashes, blisters, and skin peeling.
Lymphoma (Cutaneous T-cell Lymphoma): A type of skin cancer that can present with itchy, papular rashes, among other symptoms.

Pityriasis Lichenoides: A rare skin condition characterized by small, itchy papules that can leave scars.
Keratosis Pilaris: A condition causing small, rough, sandpapery bumps on the skin, often on the arms, thighs, cheeks, and buttocks, which can be itchy.
Darier Disease: A rare genetic disorder leading to an itchy, papular rash, particularly in seborrheic areas.
Grover's Disease: A rare skin condition causing itchy, papular rashes, often appearing on the chest and back.