Ankylosing Spondylitis

Explanation

Bilateral radiographic sacroiliitis with inflammatory back pain, restricted lumbar movement in two planes, asymmetric large-joint lower limb arthritis, and enthesitis at the Achilles insertion is ankylosing spondylitis, the prototype of axial spondyloarthritis. Enthesitis, the inflammation of tendon and ligament insertions, is the pathological hallmark of the spondyloarthritis group and distinguishes it from synovium-based arthritis. Ankylosing spondylitis usually begins in the third decade, is strongly linked to HLA-B27, and involves the sacroiliac joints bilaterally and symmetrically before ascending the spine. Reactive arthritis shares HLA-B27, enthesitis, and lower limb oligoarthritis, but it follows a gastrointestinal or genitourinary infection by 1 to 4 weeks and is usually self-limiting with unilateral or asymmetric sacroiliitis; the explicit absence of any preceding infection makes it much less likely. Rheumatoid arthritis causes a symmetric small-joint polyarthritis of the hands and feet with morning stiffness, is usually seropositive for rheumatoid factor and anti-cyclic citrullinated peptide antibodies, and spares the sacroiliac joints and lumbar spine other than the atlantoaxial junction. Crystal arthritis, whether gout or calcium pyrophosphate deposition, produces acute monoarticular or oligoarticular attacks that resolve over days rather than a 3-month inflammatory course, and this patient's urate is normal. First-line treatment is a regular non-steroidal anti-inflammatory drug with a structured exercise and physiotherapy programme, escalating to a tumour necrosis factor or interleukin-17 inhibitor when disease activity persists; conventional agents such as methotrexate do not work for axial disease. Extra-articular complications to screen for include acute anterior uveitis, inflammatory bowel disease, aortic root disease, and osteoporotic spinal fracture.

Why each option

A.

Reactive arthritis follows a gastrointestinal or genitourinary infection and typically causes asymmetric sacroiliitis; no antecedent infection is present here.

B.

Rheumatoid arthritis is a symmetric small-joint polyarthritis that spares the sacroiliac joints, and both rheumatoid factor and anti-cyclic citrullinated peptide antibodies are negative.

C.

Correct. Inflammatory back pain with restricted lumbar movement, enthesitis, and bilateral grade 3 sacroiliitis defines ankylosing spondylitis.

D.

Crystal arthritis causes acute self-limiting attacks over days with a normal spine, and this patient's serum urate is normal.

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