Lymphedema: This condition is characterized by the abnormal accumulation of protein-rich fluid in the interstitial tissue, leading to swelling. It can be primary (inherited) or secondary (acquired) due to surgery, infection, or trauma. Given the unilateral presentation, secondary lymphedema due to a localized cause (e.g., surgery, infection) is a plausible explanation.

Deep Vein Thrombosis (DVT): DVT can cause swelling, pain, and discoloration of the affected limb. The swelling is usually unilateral and can be a result of a blood clot obstructing blood flow.
Cellulitis: A bacterial infection of the skin and subcutaneous tissues, cellulitis can cause swelling, redness, warmth, and pain. It often affects one limb at a time.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and pain. It typically affects both legs but can be asymmetric.

Superior Vena Cava Syndrome: Although rare, this condition, characterized by the obstruction of the superior vena cava, can cause swelling of the face, arms, and upper body. It's crucial to identify and treat promptly due to its potential for serious complications.
Abdominal Aortic Aneurysm: An aneurysm in the abdominal aorta can compress or obstruct nearby structures, including veins, leading to unilateral swelling of the lower limb. Early detection is critical due to the risk of rupture.
Malignancy: Certain cancers, such as lymphoma or metastatic disease, can cause localized swelling due to tumor growth or lymphatic obstruction. Identifying malignancy early is vital for effective treatment.

Klippel-Trenaunay Syndrome: A rare congenital condition characterized by port-wine stains, varicose veins, and soft tissue and bone hypertrophy, leading to swelling and enlargement of the affected limb.
Parkes Weber Syndrome: Similar to Klippel-Trenaunay Syndrome, it involves congenital malformations of blood vessels and can cause swelling, but it typically includes arteriovenous fistulas.
Chronic Venous Insufficiency: While not rare in itself, the severe form that leads to significant unilateral swelling due to long-standing venous hypertension is less common and can be considered under rare diagnoses in the context of acute presentation.