Cellulitis: This is a common condition characterized by unilateral edema, redness, warmth, and sometimes purpura, typically caused by bacterial infection. The unilateral presentation and presence of purpura make cellulitis a leading consideration.

Deep Vein Thrombosis (DVT): DVT can cause unilateral edema and, in severe cases, purpura due to venous obstruction and potential hemorrhage into the skin. The risk factors and clinical presentation will guide the likelihood of this diagnosis.
Lymphedema: While typically not associated with purpura, in its acute phases or with secondary infection (cellulitis), lymphedema can present with unilateral edema. Purpura might be less common but could occur due to skin fragility or secondary infection.
Allergic Contact Dermatitis or Irritant Contact Dermatitis: These conditions can cause localized edema and, in severe cases, might lead to purpura, especially if there's a significant inflammatory response or if the skin barrier is compromised.

Necrotizing Fasciitis: Although rare, this condition is life-threatening and can initially present with unilateral edema and purpura. It requires immediate recognition and treatment.
Warfarin-Induced Skin Necrosis: This is a rare but serious condition that can occur in patients on warfarin therapy, presenting with skin necrosis and purpura, typically in areas with more fatty tissue.
Heparin-Induced Thrombocytopenia (HIT) with Skin Necrosis: Another rare condition that can present with skin lesions, including purpura, due to thrombocytopenia and paradoxical thrombosis.

Malignancy (e.g., Lymphoma or Leukemia): Certain malignancies can cause localized edema and purpura due to infiltration of the skin or due to paraneoplastic syndromes.
Vasculitis (e.g., Henoch-Schönlein Purpura, Granulomatosis with Polyangiitis): These conditions can present with purpura and, in some cases, localized edema, depending on the vessels involved and the severity of the disease.
Sickle Cell Disease with Sickle Cell Crisis: While more commonly associated with systemic symptoms, sickle cell crisis can lead to localized tissue ischemia, potentially causing edema and purpura in affected areas.