A 32-year-old man presents with a 6-month history of insidious low back pain and morning stiffness lasting 45 minutes. He reports alternating buttock pain and notes that his symptoms improve with exercise but worsen with rest. On examination, he has reduced lumbar spine mobility. An AP pelvis X-ray is obtained. The structure marked **B** in the diagram shows bilateral sclerosis and early fusion. Which of the following is the most likely diagnosis?

Explanation

## Why Ankylosing spondylitis with HLA-B27 positivity is right Ankylosing spondylitis (AS) is the prototype spondyloarthropathy characterized by chronic inflammatory axial skeleton involvement. The sacroiliac (SI) joint, marked **B**, is a synovial joint anteriorly and fibrous posteriorly—predominantly weight-bearing with minimal mobility. Sacroiliitis (bilateral sclerosis and fusion of the SI joints) is the earliest and most characteristic radiographic finding in AS, often preceding vertebral fusion. The clinical presentation—insidious onset before age 40, morning stiffness >30 minutes, alternating buttock pain (sacroiliitis), and improvement with exercise—is pathognomonic for inflammatory back pain in AS. HLA-B27 is positive in 90% of AS patients (vs 8% in the general population), making it highly sensitive though not diagnostic in isolation. The ASAS criteria require sacroiliitis on imaging plus ≥1 spondyloarthropathy feature or HLA-B27 positivity plus ≥2 features—this patient meets both pathways. [Robbins 10e Ch 26; Harrison 21e Ch 366] ## Why each distractor is wrong - **Mechanical low back pain with facet joint osteoarthritis**: Mechanical back pain typically has acute or subacute onset, worsens with activity/exercise, improves with rest, and lacks morning stiffness >30 minutes. The SI joint involvement with bilateral sclerosis is not a feature of mechanical disease; it is pathognomonic for inflammatory spondyloarthropathy. - **Lumbar disc herniation with radiculopathy**: Disc herniation presents with radicular pain (leg pain following a dermatomal distribution), not buttock pain, and does not cause bilateral SI joint sclerosis or morning stiffness. Imaging would show disc prolapse, not SI joint pathology. - **Rheumatoid arthritis with axial involvement**: Rheumatoid arthritis is HLA-DR4/DR1-associated (not HLA-B27), primarily affects peripheral small joints (hands, feet) symmetrically, and does not characteristically involve the SI joints or cause sacroiliitis. Axial involvement in RA is rare and late; it is not the presenting feature. **High-Yield:** Bilateral sacroiliitis on X-ray (sclerosis and fusion of SI joints marked **B**) is the earliest radiographic hallmark of ankylosing spondylitis; combined with inflammatory back pain pattern and HLA-B27, it is diagnostic. [Robbins 10e Ch 26; Harrison 21e Ch 366]