Deep Vein Thrombosis (DVT): This is the most common cause of unilateral lower extremity edema, especially in the absence of trauma or surgery. DVT occurs when a blood clot forms in the deep veins of the leg, leading to swelling, pain, and warmth.

Cellulitis: A bacterial infection of the skin and subcutaneous tissue, which can cause edema, redness, and warmth in one leg.
Lymphedema: A condition characterized by the abnormal collection of protein-rich fluid in the interstitial tissue, leading to swelling of one leg.
Chronic Venous Insufficiency: A condition where the veins have problems returning blood from the legs to the heart, leading to edema, varicose veins, and skin changes.

Compartment Syndrome: A serious condition where increased pressure within a muscle compartment leads to nerve and muscle damage, which can cause edema, pain, and paresthesia in one leg.
Peripheral Artery Disease (PAD): A condition where narrowed arteries reduce blood flow to the legs, leading to edema, pain, and weakness in one leg.
Phlegmasia Cerulea Dolens: A rare but life-threatening condition where a large DVT causes severe swelling and cyanosis of the affected leg.

Lipedema: A rare condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and pain in one leg.
Filarial Elephantiasis: A parasitic infection that can cause severe swelling and lymphedema in one leg, typically seen in tropical regions.
Klippel-Trenaunay Syndrome: A rare congenital condition characterized by abnormal blood and lymphatic vessels, leading to edema, varicose veins, and skin changes in one leg.