A 38-year-old man from rural India presents with a 6-month history of progressive back pain, low-grade fever, and night sweats. On examination, he has kyphotic deformity of the thoracic spine and neurological examination reveals early signs of spastic paraparesis. Plain radiographs show anterior vertebral body collapse with preservation of the intervertebral disc space. What is the investigation of choice to confirm the diagnosis of Pott disease?