Ankle Sprain: This is the most common cause of swelling in the exterior ankle, typically resulting from inversion or eversion injuries that stretch or tear the ligaments surrounding the ankle joint. The mechanism of injury and the location of the swelling and pain are key indicators.

Peroneal Tendinitis: Inflammation of the peroneal tendons, which run along the outside of the ankle, can cause swelling and pain in the exterior ankle area. This condition is often related to overuse or repetitive ankle motion.
Ankle Fracture: A fracture of one of the bones in the ankle can cause significant swelling, especially if the fracture is displaced. The severity of the pain and the inability to bear weight are clues to this diagnosis.
Gout or Pseudogout: These are forms of arthritis that can cause sudden, severe swelling and pain in the ankle, among other joints. A history of similar episodes or the presence of urate crystals in the joint fluid can help diagnose gout.

Compartment Syndrome: A condition where increased pressure within a muscle compartment (in this case, the compartments of the lower leg) leads to inadequate blood supply to the muscles and nerves. This is a medical emergency that requires prompt diagnosis and treatment to prevent permanent damage.
Deep Vein Thrombosis (DVT): Although more commonly associated with swelling in the calf, DVT can occasionally present with swelling in the ankle area. Given the risk of pulmonary embolism, it's crucial not to miss this diagnosis, especially in patients with risk factors for DVT.
Osteomyelitis or Septic Arthritis: Infections of the bone or joint, respectively, can cause swelling, redness, and pain in the affected area. These conditions are medical emergencies, particularly in immunocompromised patients or those with a history of recent trauma or infection.

Erythromelalgia: A rare disorder characterized by burning pain, heat, and redness in the hands and feet, which can sometimes include the ankles. It's more common in patients with certain underlying conditions, such as myeloproliferative disorders.
Complex Regional Pain Syndrome (CRPS): A chronic pain condition that most often affects one of the arms, legs, hands, or feet after an injury or trauma. It can cause swelling, among other symptoms, and is diagnosed based on clinical presentation and exclusion of other causes.
Soft Tissue Tumors: Rarely, swelling in the exterior ankle could be due to a soft tissue tumor, such as a lipoma, cyst, or, very rarely, a malignant tumor like a sarcoma. These would typically have a more gradual onset and might be associated with a palpable mass.