Deep Vein Thrombosis (DVT): This is the most likely diagnosis given the combination of a swollen lower extremity and an elevated D-dimer. DVT is a common condition where a blood clot forms in the deep veins of the body, often in the legs. The elevated D-dimer, a protein fragment produced when a blood clot dissolves, supports the presence of a thrombotic event.

Cellulitis: While cellulitis is an infection of the skin and subcutaneous tissues and does not directly cause an elevated D-dimer, it can cause significant swelling and inflammation of the affected limb. In some cases, especially if there's a concomitant condition or complication, D-dimer might be elevated due to associated inflammation or a secondary thrombotic event.
Lymphedema: This condition involves swelling of the limbs due to impaired lymph drainage. Although it doesn't typically cause an elevated D-dimer, in advanced stages or with secondary complications, there might be an overlap in presentation.
Muscle Strain or Trauma: Significant trauma to the lower extremity can cause swelling and, in some cases, lead to an elevation in D-dimer levels due to muscle damage or associated thrombosis.

Pulmonary Embolism (PE): Although the primary symptom of PE is typically respiratory distress, a significant DVT can lead to a PE if the clot breaks loose and travels to the lungs. This is a potentially life-threatening condition that must be considered, especially with an elevated D-dimer.
Phlegmasia Cerulea Dolens: A severe form of DVT that causes significant swelling and cyanosis of the affected limb due to obstruction of venous outflow. It's a medical emergency that requires prompt intervention.

Superficial Thrombophlebitis: Inflammation of the superficial veins, which can cause swelling and pain but is less likely to cause a significant elevation in D-dimer unless it's extensive or there's an underlying hypercoagulable state.
Klippel-Trenaunay Syndrome: A rare congenital condition characterized by abnormal development of blood vessels, leading to swelling and other complications in the affected limb. It might present with elevated D-dimer levels in the context of thrombotic complications.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling. While not typically associated with elevated D-dimer, it could be considered in the differential for lower extremity swelling, albeit less likely to be directly related to the D-dimer elevation.