Vascular Insufficiency: This is the most likely cause of a dark dorsal extremity, as it can lead to reduced blood flow and oxygenation to the affected area, resulting in discoloration.

Diabetic Neuropathy: High blood sugar levels can damage nerves, leading to decreased sensation and blood flow, which may cause discoloration of the extremities.
Chronic Venous Insufficiency: This condition can cause blood to pool in the legs, leading to increased pressure and potential discoloration.
Hemosiderin Deposits: Breakdown of red blood cells can lead to the accumulation of hemosiderin, causing skin discoloration.

Compartment Syndrome: A serious condition that can lead to nerve and muscle damage if not promptly treated, characterized by increased pressure within a muscle compartment.
Deep Vein Thrombosis (DVT): A blood clot in the deep veins can cause discoloration, pain, and swelling, and can be life-threatening if it breaks loose and travels to the lungs.
Sepsis: A severe infection that can cause widespread inflammation and organ failure, potentially leading to discoloration of the extremities.

Erythromelalgia: A rare disorder characterized by burning pain, heat, and redness in the hands and feet, which can sometimes cause discoloration.
Argyria: A rare condition caused by exposure to silver or silver compounds, leading to blue-gray discoloration of the skin.
Chronic Arsenic Poisoning: Long-term exposure to arsenic can cause skin discoloration, among other symptoms.