Iliotibial band syndrome

A 33-year-old man presents to the clinic with worsening knee pain for the past 5 months. He is currently training for a marathon, and the pain is brought on by running. His past medical history is unremarkable, and he does not take any medications. Physical examination is notable for tenderness to palpation over the lateral knee superior to the joint line at the lateral femoral epicondyle. There is no joint swelling of the knee and no gapping at the knee with valgus or varus testing. In addition, a test is performed that results in failure of the hip to adduct and lateral knee pain when the patient is lying with the affected side up and the distal leg is held with the hip abducted and extended and then dropped until motion is restricted. Which of the following is the most likely diagnosis?