Deep Vein Thrombosis (DVT): This is the most likely diagnosis given the combination of elevated D-dimer and leg pain. 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 clot.

Pulmonary Embolism (PE): Although PE typically presents with respiratory symptoms, it can also cause leg pain if it originates from a DVT. An elevated D-dimer is common in PE.
Cellulitis: This is an infection of the skin and subcutaneous tissues. While it doesn't directly cause an elevated D-dimer, severe infections can lead to a systemic inflammatory response that might elevate D-dimer levels. However, this would be less common.
Muscle Strain or Trauma: Significant muscle injury can elevate D-dimer levels due to the release of tissue factor and subsequent activation of the coagulation cascade.

Aortic Dissection: Although less common, an aortic dissection can cause leg pain and might elevate D-dimer levels due to the involvement of the aortic wall and potential activation of coagulation pathways. Missing this diagnosis can be fatal.
Severe Infections (e.g., Septic Thrombophlebitis): Certain infections can cause both elevated D-dimer levels and leg pain, especially if there's involvement of the venous system.

Malignancy: Some malignancies can increase the risk of thrombosis (Trousseau’s syndrome) and thus could present with elevated D-dimer and leg pain due to DVT or PE.
Antiphospholipid Syndrome: This autoimmune disorder can cause blood clots in both arteries and veins, leading to elevated D-dimer levels and potentially leg pain if a DVT is present.
Thromboangiitis Obliterans (Buerger's Disease): A rare disease of the blood vessels in the arms and legs, which can cause pain and might lead to elevated D-dimer levels due to chronic inflammation and clotting.