Iliotibial (IT) Band Syndrome: This condition is characterized by inflammation of the iliotibial band, a ligament that runs down the outside of the thigh from the hip to the knee. It is a common cause of lateral knee pain and swelling, especially in runners or cyclists, due to repetitive friction of the IT band over the lateral femoral condyle.

Lateral Meniscal Tear: Tears in the lateral meniscus can cause swelling and pain on the lateral aspect of the knee. Meniscal tears are common in sports that involve twisting and pivoting.
Lateral Collateral Ligament (LCL) Sprain: An injury to the LCL, which connects the femur to the fibula, can lead to lateral knee swelling and instability.
Bursitis (e.g., IT band bursitis or lateral gastrocnemius bursitis): Inflammation of the bursae around the knee can cause localized swelling and pain.

Septic Arthritis: Although less common, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with joint swelling, pain, and systemic symptoms like fever.
Osteonecrosis (Avascular Necrosis) of the Lateral Femoral Condyle: This condition involves the death of bone tissue due to a lack of blood supply and can cause pain and swelling in the affected area.
Tumor (e.g., osteosarcoma, synovial sarcoma): Although rare, tumors around the knee can cause swelling and should be considered in the differential diagnosis to ensure timely treatment.

Ganglion Cyst: A benign lump that can occur on the lateral aspect of the knee, often associated with a tendon or joint.
Pigmented Villonodular Synovitis (PVNS): A rare condition characterized by overgrowth of the joint lining, leading to swelling and pain.
Hemophilia or other bleeding disorders: In patients with bleeding disorders, even minor trauma can lead to significant hemarthrosis (bleeding into the joint), causing swelling and pain.