Ankle Sprain: This is often the most common cause of unilateral foot swelling and pain, especially if there's a history of recent trauma or injury to the ankle. The sprain can cause inflammation, leading to swelling and pain.

Cellulitis: A bacterial skin infection that can cause redness, swelling, and pain in the affected area. It's more likely if there's a break in the skin or if the patient has a history of similar infections.
Deep Vein Thrombosis (DVT): Although more common in the legs, DVT can cause swelling and pain in one foot, especially if the clot is located near the ankle or foot.
Gout: A form of arthritis characterized by sudden, severe attacks of pain, swelling, redness, and tenderness in one or more joints, most often in the big toe.
Plantar Fasciitis: Inflammation of the plantar fascia, a band of tissue that runs along the bottom of the foot, causing pain and swelling, especially with weight-bearing activities.

Osteomyelitis: A bone infection that can cause pain, swelling, and redness in the affected area. It's crucial to diagnose and treat promptly to prevent serious complications.
Compartment Syndrome: A condition where pressure within the muscles builds to dangerous levels, cutting off blood flow and preventing nourishment and oxygen from reaching nerve and muscle cells. It's a medical emergency.
Septic Arthritis: A joint infection that can cause severe pain, swelling, and redness. Prompt diagnosis and treatment are essential to prevent joint destruction and other complications.

Erythromelalgia: A rare disorder characterized by burning pain, heat, and redness in the hands and feet, which can also cause swelling.
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's characterized by intense burning or aching pain, along with swelling and changes in skin temperature and color.
Lymphedema: A condition characterized by swelling in one or more extremities, resulting from impaired flow in the lymphatic system. It's less common in the foot but can occur, especially if there's a history of surgery, infection, or trauma affecting lymph nodes.