Matthew, 16, is at your office today for his mandatory physical prior to soccer tryouts next month. You've known him for several years now as a perfectly healthy and courteous young man.

The examination proceeds as usual, and you find nothing out of the ordinary until you test his patellar reflex. To your surprise, you notice Matthew grimacing in pain as you examine his right knee.

The skin overlying the limb appears normal, and there are no gross deformities, or palpable masses. However, you do note some stiffness and pain on extension.