Ankle Sprain: This is the most common cause of soft tissue swelling around the ankle, resulting from ligamentous injury, often due to inversion or eversion of the ankle. The mechanism of injury and the presence of pain, swelling, and limited mobility support this diagnosis.

Cellulitis: A bacterial infection of the skin and soft tissues, characterized by redness, warmth, swelling, and sometimes fever. It can occur around the ankle, especially if there's a break in the skin.
Achilles Tendinitis: Inflammation of the Achilles tendon, leading to pain and swelling at the back of the ankle. This condition is common in runners or individuals who suddenly increase their physical activity.
Peroneal Tendinitis: Inflammation of the peroneal tendons, which run along the outside of the ankle, causing pain and swelling in this area. Overuse or ankle sprains can lead to this condition.

Compartment Syndrome: A serious condition resulting from increased pressure within a muscle compartment, leading to pain, swelling, and potentially permanent damage if not promptly treated. It's crucial to identify this condition early due to its potential for severe complications.
Deep Vein Thrombosis (DVT): A blood clot in the deep veins, which can cause swelling, warmth, and discoloration of the affected limb. Although less common in the ankle, DVT is a critical diagnosis to consider due to its risk of pulmonary embolism.
Osteomyelitis: An infection of the bone, which can cause pain, swelling, and redness over the affected bone. In the context of the ankle, this could be a complication of a puncture wound or surgery.
Septic Arthritis: A bacterial infection within a joint, leading to severe pain, swelling, and decreased range of motion. Prompt diagnosis and treatment are essential to prevent joint destruction.

Erythema Nodosum: An inflammatory condition characterized by the formation of nodules under the skin, often on the lower legs, including around the ankles. It can be associated with various systemic conditions or infections.
Gout or Pseudogout: Forms of arthritis that can cause sudden, severe joint pain and swelling, including in the ankle. While more commonly associated with the big toe, these conditions can affect any joint.
Soft Tissue Sarcoma: A rare type of cancer that originates in the soft tissues, including muscles, tendons, and ligaments. Although extremely uncommon, any unexplained, persistent swelling should be thoroughly investigated.