Iliotibial (IT) Band Syndrome: This condition is characterized by pain and tenderness on the lateral aspect of the knee, particularly above the joint line and over the lateral femoral condyle. The IT band crosses the lateral femoral condyle at an angle of about 30 degrees of knee flexion, which can cause friction and pain during running, especially when the knee is flexed. The symptoms described, including pain at the end of runs, at rest, and with passive flexion of the knee producing pain at 30 degrees of flexion, are classic for IT band syndrome.

Lateral Meniscus Tear: Tears in the lateral meniscus can cause pain on the lateral side of the knee. Although the primary complaint is lateral knee pain, meniscal tears typically cause mechanical symptoms such as locking, catching, or giving way, which are not mentioned. However, it remains a plausible diagnosis given the location of pain.
Lateral Collateral Ligament (LCL) Sprain: An LCL sprain could cause lateral knee pain and tenderness. This would typically follow a traumatic event or a valgus stress to the knee, which is not specified in the scenario provided.
Popliteus Tendinitis: The popliteus tendon is located on the lateral aspect of the knee and can cause pain with activities that involve knee flexion and external rotation. While less common, it could be considered given the location and nature of the pain.

Osteonecrosis of the Femoral Condyle: Although rare, osteonecrosis (avascular necrosis) of the femoral condyle can present with knee pain and should be considered, especially if the pain is severe and at rest. Missing this diagnosis could lead to significant morbidity.
Bone Tumor: Tumors such as osteosarcoma or metastatic disease can present with localized pain and should always be considered in the differential diagnosis of persistent or severe pain, even though they are rare.
Infection (Septic Arthritis or Osteomyelitis): Infection within the knee joint (septic arthritis) or bone (osteomyelitis) is a medical emergency. While the presentation might not typically include fever or systemic symptoms initially, any suspicion of infection warrants immediate investigation.

Nerve Entrapment (Peroneal Nerve): The peroneal nerve wraps around the lateral aspect of the fibular head and can be entrapped, causing pain and sometimes weakness in the foot. This would be an unusual cause of the described symptoms but could be considered if other diagnoses are ruled out.
Fabella Syndrome: A fabella (small sesamoid bone) located in the lateral head of the gastrocnemius can cause pain if it becomes inflamed or if the surrounding tissue becomes irritated. This is a rare condition and would be considered only after more common causes are excluded.