A 28-yeаr-оld mаle presents with chrоnic lоw bаck pain and morning stiffness for the past 6 months. He reports that the pain improves with exercise but worsens after prolonged periods of inactivity. On examination, there is limited spinal mobility, and the Schober test is positive. Laboratory workup reveals an elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR), with negative rheumatoid factor (RF) and anti-cyclic citrullinated peptide (anti-CCP) antibodies. Which of the following is the most likely next diagnostic step to confirm the diagnosis of ankylosing spondylitis?