Chronic Venous Insufficiency: This condition is characterized by the veins having problems returning blood from the legs to the heart due to valve issues, leading to swelling, pain, and potentially skin changes. It's a common cause of leg swelling and could easily involve the knee area.

Deep Vein Thrombosis (DVT): Although DVT typically presents with acute onset of swelling and pain, it's possible for it to be less acute and still cause significant symptoms. The knee area can be involved, especially if the clot is in the proximal veins.
Cellulitis: A bacterial infection of the skin and subcutaneous tissues, which can cause swelling, redness, warmth, and pain. The knee area can be affected, and it's a common enough condition to be considered in the differential.
Osteoarthritis of the Knee: While primarily causing joint pain, osteoarthritis can lead to swelling around the knee due to inflammation. It's a very common condition, especially in older adults.

Pulmonary Embolism (if DVT is present): If DVT is suspected, it's crucial to consider the risk of pulmonary embolism, a potentially life-threatening condition where the clot breaks loose and travels to the lungs.
Septic Arthritis: An infection within the joint space, which is a medical emergency. It can cause severe pain, swelling, and warmth around the joint and can lead to rapid destruction of the joint if not promptly treated.
Necrotizing Fasciitis: A severe infection of the fascia, the tissue surrounding muscles, nerves, fat, and blood vessels. It's rare but can be life-threatening and requires immediate medical attention.

Lymphedema: A condition characterized by chronic swelling due to a lymphatic system dysfunction. It's less common than venous insufficiency but can cause significant swelling and discomfort.
Knee Tumor (e.g., Osteosarcoma): Although rare, tumors around the knee can cause swelling and pain. They would be an unusual cause of these symptoms but should be considered, especially if other diagnoses have been ruled out.