Chronic Venous Insufficiency: This condition is the most common cause of leg swelling, especially in older adults or those with a history of deep vein thrombosis. It occurs when the veins have problems returning blood from the legs back to the heart due to issues like faulty valves.

Heart Failure: Leg swelling can be a symptom of heart failure, where the heart is not pumping efficiently, leading to fluid buildup in the legs.
Kidney Disease: Conditions like nephrotic syndrome can cause leg swelling due to the loss of protein in the urine, leading to low albumin levels and fluid retention.
Liver Disease (Cirrhosis): Cirrhosis can lead to fluid retention and swelling in the legs due to hypoalbuminemia and portal hypertension.
Lymphedema: This is a condition characterized by the abnormal collection of protein-rich fluid in the interstitial tissue, leading to swelling of one or more extremities.

Deep Vein Thrombosis (DVT): Although not the most common cause, DVT is a critical diagnosis to consider due to its potential for severe complications like pulmonary embolism.
Cellulitis: A bacterial infection of the skin and subcutaneous tissues that can cause swelling, redness, and warmth of the affected leg.
Compartment Syndrome: A condition where increased pressure within a muscle compartment leads to inadequate blood supply to the tissues inside the compartment, which is a medical emergency.

Pulmonary Embolism with Associated Leg Swelling: While pulmonary embolism itself does not directly cause leg swelling, it can be associated with DVT, which does.
Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the lymph nodes, which might lead to swelling in the legs.
Fabry Disease: A rare genetic disorder that can lead to the accumulation of a particular type of fat in the body's cells, affecting many parts of the body and potentially causing leg swelling.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and pain.