Iliotibial (IT) Band Syndrome: This condition is common in active individuals, especially runners or cyclists, due to repetitive knee flexion and extension, causing friction and irritation of the iliotibial band as it moves over the lateral femoral condyle.

Lateral Meniscus Tear: Meniscal tears can occur from acute injuries or chronic wear and tear, leading to pain and dysfunction, especially with activities that involve twisting or pivoting.
Popliteus Tendinitis: Inflammation of the popliteus tendon, which helps to stabilize the knee, can cause posterior-lateral knee pain, especially with activities that involve knee rotation.
Peroneal Tendinitis: Though more commonly associated with ankle issues, peroneal tendinitis can refer pain to the lateral aspect of the knee due to the anatomical course of the peroneal tendons.

Deep Vein Thrombosis (DVT): While less common, DVT can present with pain in the leg, including the posterior-lateral aspect of the knee, and is a medical emergency due to the risk of pulmonary embolism.
Osteonecrosis of the Femoral Condyle: A condition where the blood supply to the bone is interrupted, leading to bone death. It can cause severe knee pain and is critical to diagnose early to prevent further bone collapse.
Tumor: Though rare, bone or soft tissue tumors can cause localized pain and must be considered in the differential diagnosis to ensure timely and appropriate treatment.

Baker's Cyst Rupture: A Baker's cyst is a fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee. While not typically posterior-lateral, a ruptured Baker's cyst can cause pain in this area due to the spread of fluid.
Ganglion Cyst: These are lumps that can form below the surface of the skin, often around joints or tendons, and can cause pain and discomfort if they press on nearby nerves.
Nerve Entrapment Syndromes: Such as peroneal nerve entrapment, which can cause pain, numbness, and weakness in the lateral aspect of the lower leg and knee area.