A six-year-old hunter-jumper horse is presented with a three-month history of poor performance and intermittent shifting hindleg lameness.
There is reluctance to bring the hind limbs forward at a trot and he occasionally drags his hind toes.
On physical exam, there is poor muscling of the gluteal muscles and some asymmetry to the croup (rump). The horse shows pain and a reluctance to ventroflex the back when midline pressure is applied.
Which one of the following is a top differential diagnosis?