Lymphedema: This condition is characterized by the abnormal collection 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 affecting the lymph nodes. Given its relatively common occurrence and the specificity of swelling to one side of the body, lymphedema is a plausible initial consideration.

Deep Vein Thrombosis (DVT): Although DVT typically presents with unilateral leg swelling, it can also cause swelling in other parts of the body if the thrombosis is extensive or if there are multiple clots. The swelling is usually accompanied by pain, warmth, and discoloration of the affected limb.
Cellulitis: A bacterial infection of the skin and subcutaneous tissues, cellulitis can cause localized swelling, redness, warmth, and pain. It often affects one limb or area of the body and can be a common cause of unilateral swelling.
Venous Insufficiency: This condition occurs when the veins have problems returning blood from the body back to the heart due to valve issues or blockage, leading to fluid accumulation and swelling, typically in the legs.

Superior Vena Cava Syndrome: Although rare, this condition, characterized by the obstruction of the superior vena cava, can lead to swelling of the face, neck, and upper limbs. It's often associated with malignancies or thrombosis and is critical to diagnose early due to its potential for severe complications.
Nephrotic Syndrome: Characterized by severe proteinuria, hypoalbuminemia, and edema, nephrotic syndrome can cause significant swelling. While it typically presents with bilateral edema, in some cases, the swelling might appear more pronounced on one side due to positional factors or underlying anatomical issues.
Heart Failure: Specifically, right-sided heart failure can lead to peripheral edema due to the backup of blood into the body's tissues. While it usually causes bilateral swelling, unilateral swelling can occur, especially if there are pre-existing venous or lymphatic abnormalities.

Klippel-Trenaunay Syndrome: A rare congenital malformation that involves blood and lymph vessels, leading to abnormal growth and swelling of the affected limb.
Proteus Syndrome: A rare genetic disorder characterized by an overgrowth of the skin, bones, and organs, which can lead to asymmetric swelling among other symptoms.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and pain, typically affecting women. It's often mistaken for lymphedema but does not typically involve the feet.