Cellulitis: This is a common cause of unilateral ankle swelling and pain, often accompanied by redness and warmth of the skin. It's usually caused by a bacterial infection.

Deep Vein Thrombosis (DVT): DVT can cause swelling, pain, and warmth in one leg. It's a blood clot in a deep vein, typically in the legs.
Sprain or Strain: Ankle injuries can lead to swelling and pain, especially if the ligaments or muscles are stretched or torn.
Gout: This form of arthritis can cause sudden, severe pain and swelling in the ankle, often accompanied by redness and warmth.
Osteoarthritis: While less common in the ankle compared to other joints, osteoarthritis can cause pain and swelling, especially if there's been a previous injury.

Compartment Syndrome: A serious condition that occurs when pressure within the muscles builds to dangerous levels, reducing blood flow. It's characterized by severe pain, especially on palpation, and can lead to permanent damage if not promptly treated.
Septic Arthritis: A bacterial infection within a joint, which is a medical emergency requiring immediate attention to prevent joint destruction.
Necrotizing Fasciitis: A severe infection of the tissue beneath the skin, including fat and the tissue covering the muscles (fascia). It's much rarer than cellulitis but can be life-threatening.

Erythromelalgia: A disorder characterized by burning pain, heat, and redness in the hands and feet, which can sometimes present with swelling.
Lymphedema: Swelling caused by impaired lymph drainage, which can be primary (inherited) or secondary (due to another condition or surgery).
Pseudogout (Calcium Pyrophosphate Deposition Disease, CPPD): A type of arthritis that causes sudden, severe joint pain and swelling, similar to gout, but caused by calcium pyrophosphate dihydrate crystal deposition.
Osteomyelitis: An infection of the bone, which can cause pain, swelling, and redness over the affected bone, and is usually accompanied by systemic symptoms like fever.