A 13-year-old who has played basketball four evenings a week since he was nine is brought in by his mother, who booked the appointment because of what he has stopped doing rather than anything he has said. He has dropped from the starting five to the bench, sits out the warm-up sprints, and has started walking up the stairs at home one at a time. It has been three months, with no single day it began, and there was no fall, collision or twist that anyone remembers. He looks well and mildly embarrassed to be here, says it is "not that bad" twice, and volunteers nothing — every detail comes from his mother or from being asked directly.