30th April 2024

A 30-year-old male presents with a two-year history of lower back pain, which is worse in the mornings and improves with exercise.

A 30-year-old male presents with a two-year history of lower back pain, which is worse in the mornings and improves with exercise. He has also noticed mild stiffness in his neck. He denies any eye pain or changes in vision. On examination, there is limited flexion in the lumbar spine and reduced chest expansion.

Investigations:

Haemoglobin: 135 g/L (130–170)

White cell count: 7.2 × 10⁹/L (4.0–11.0)

CRP: 15 mg/L (<5)

HLA-B27: Positive

Which is the most appropriate next investigation?