Cellulitis or Deep Vein Thrombosis (DVT) with secondary skin manifestations: The combination of non-blanching purpura, localized edema, and pruritus could be indicative of an infectious or thrombotic process affecting the right arm. Cellulitis could cause purpura and edema, while DVT might lead to secondary skin changes including purpura due to venous stasis.

Vasculitis (e.g., Henoch-Schönlein Purpura, Leukocytoclastic Vasculitis): These conditions can cause non-blanching purpura and may be associated with edema and pruritus. Vasculitis often involves an autoimmune response and can affect various organs.
Lymphatic Obstruction or Lymphedema: This could explain the edema and potentially the pruritus if the skin is stretched or irritated. Non-blanching purpura might be less common but could occur due to secondary changes.
Allergic Contact Dermatitis or Irritant Contact Dermatitis: While these conditions primarily cause pruritus and dermatitis, severe cases could potentially lead to secondary changes that might resemble non-blanching purpura, especially if there's significant inflammation or if the patient scratches excessively.

Malignancy (e.g., Lymphoma, Leukemia): Certain malignancies can cause a wide range of skin manifestations, including non-blanching purpura, due to either direct infiltration of the skin or secondary effects like coagulopathy or vasculitis.
Infectious Endocarditis: Although less common, infectious endocarditis can cause non-blanching purpura (Janeway lesions) and might be associated with embolic phenomena leading to localized edema.
Thrombocytopenia or Coagulopathy: Conditions affecting platelet count or function, or the coagulation cascade, can lead to purpura. Edema might not be directly related but could be part of a broader clinical picture.

Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. While primarily known for skin hyperextensibility and joint hypermobility, some types can lead to easy bruising and poor wound healing.
Scurvy (Vitamin C Deficiency): Can cause perifollicular petechiae and purpura, along with edema and other systemic symptoms. Pruritus might be less common but could occur due to skin fragility and irritation.
Amyloidosis: A condition characterized by the deposition of amyloid (a particular type of protein) in various tissues. It can cause purpura, especially when associated with minor trauma, and might lead to edema due to renal involvement or other mechanisms.