A 22 year old male presents with complaints of dull lower back pain and morning stiffness. The pain was initially episodic, but now has become persistent and bilateral. On physical examination, there is tenderness over the costosternal junctions, spinous processes of the veebrae, and the iliac crests. Which of the following tests would be most likely to be helpful in establishing a diagnosis of ankylosing spondylitis?

Ankylosing spondylitis is one of the spondyloahropathies. It is a chronic inflammatory joint disease primarily affecting the veebrae and the sacroiliac joints, usually beginning in late adolescence or early adulthood. Close to 90% of Caucasian patients with ankylosing spondylitis are HLA-B27 positive, so tests for this HLA type are the most helpful of those listed in establishing the diagnosis. C-reactive protein and erythrocyte sedimentation rate are non-specific markers of inflammation that can be elevated in active ankylosing spondylitis. Serum alkaline phosphatase and serum IgA can also be (usually mildly) elevated, but do not specifically suggest ankylosing spondylitis. Ref: Qubti M.A., Flynn J.A. (2013). Chapter 17. Ankylosing Spondylitis & the Ahritis of Inflammatory Bowel Disease. In J.B. Imboden, D.B. Hellmann, J.H. Stone (Eds), CURRENT Diagnosis & Treatment: Rheumatology, 3e.