Hip dysplasia
A hip joint that formed loosely, so the ball and socket grind instead of gliding — one of the most-screened inherited problems in medium and large breeds.
The hip is a ball and socket. In a dysplastic hip the socket is too shallow or the ligament too slack, so the joint moves in ways it was not built for. The grinding wears cartilage away and arthritis follows, sometimes years before the dog is old enough for anyone to blame age.
It is partly inherited and partly shaped by growth: how fast a puppy grows, how much it weighs and what surfaces it lands on all matter. This is why responsible breeders score hips by X-ray under registry schemes such as the Orthopedic Foundation for Animals, and why a breeder who cannot show you scores for both parents is worth a second question.