Knee pain · no popping · no previous trauma · Dx?

A 36-year-old man sought care at our family medicine clinic for knee pain that he'd had for the past year. He denied any previous injury or trauma to the knee. The pain affected the posterolateral left knee and was aggravated by squatting and deep flexion. Daily activities did not bother him, but skiing, golfing, mountain biking, and lifting weights worsened the pain. His pain had gradually become more severe and frequent. He denied any mechanical symptoms such as catching, popping, or locking.