Deep Vein Thrombosis (DVT): This condition is characterized by the formation of a blood clot in a deep vein, typically in the legs. Symptoms include leg pain, swelling, discoloration, and stiffness, which match the presentation described. DVT is a common cause of these symptoms and is often considered first due to its frequency and potential for serious complications.

Cellulitis: An infection of the skin and subcutaneous tissues, cellulitis can cause pain, swelling, redness, and warmth in the affected leg. While it doesn't typically cause stiffness in the same way DVT does, the other symptoms overlap.
Chronic Venous Insufficiency: This condition occurs when the veins have problems returning blood from the legs back to the heart due to issues like faulty valves. It can lead to swelling, skin changes, and pain, especially after standing for long periods.
Peripheral Artery Disease (PAD): PAD involves the narrowing of the peripheral arteries, which can lead to pain in the legs during exercise (claudication) and at rest, especially if severe. While it primarily affects the arterial system, it can cause similar symptoms due to reduced blood flow.

Phlegmasia Cerulea Dolens: A severe form of DVT that causes significant swelling and cyanosis (blue discoloration) of the leg. It's a medical emergency due to the high risk of limb loss and pulmonary embolism.
Compartment Syndrome: A condition where increased pressure within a muscle compartment leads to insufficient blood supply to the tissues inside the compartment. It's characterized by the "5 Ps": pain, pallor, pulselessness, paresthesia, and poikilothermia. It's a surgical emergency.
Pulmonary Embolism (in the context of DVT): While not directly causing leg pain, a pulmonary embolism can occur if a DVT dislodges and travels to the lungs. It's crucial to consider this possibility, especially if the patient shows signs of respiratory distress or sudden onset of chest pain.

Erythromelalgia: A disorder characterized by burning pain, heat, and redness in the hands and feet. It can be primary or secondary to other conditions like myeloproliferative disorders.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and pain. It's often mistaken for lymphedema but does not typically cause stiffness.
Lymphedema: Swelling caused by impaired lymph drainage. It can lead to chronic swelling, pain, and stiffness in the affected limb, but the onset is usually more gradual than what is described for DVT or other acute conditions.