What is your neurologic diagnosis?

A 4-year-old 24-kg (52.8-lb) neutered female Bearded Collie was evaluated because of progressive tetraparesis. Six weeks prior to hospital admission, the dog developed thoracolumbar hyperesthesia and an abnormal gait in the pelvic limbs that resolved with NSAID treatment (meloxicam, 0.05 mg/kg [0.02 mg/lb], PO, q 24 h). Sudden-onset nonambulatory tetraparesis had developed 24 hours prior to admission. The pelvic limbs were markedly more affected with only mild voluntary movements. When provided with pelvic limb support, the dog could ambulate with a short thoracic limb stride. Results of a complete physical examination were unremarkable, other than the neurologic examination findings. The dog had been admitted to the hospital at night, and by the following morning, the tetraparesis had progressed