This page reveals the answer
Want to test yourself first? Play the original case before reading the clues and explanation below.
Five progressive clues
These are the case findings in their original reveal order, moving from the broader presentation to the most discriminating evidence.
Clue 1
A 26-year-old man, a junior accountant and weekend cyclist, comes to his primary care clinic frustrated by eight months of low back and buttock pain he first blamed on long hours at his desk. He mentions almost in passing that the ache eases once he gets moving on his morning bike ride and is at its worst after he has been sitting or lying still.
Clue 2
The pain began gradually, with no injury, and has never fully gone away in over eight months. It is deep and dull, worse in the second half of the night — he often wakes around 4 a.m. and has to walk it off — and comes with more than an hour of back stiffness each morning. Rest makes it worse; activity makes it better. A trial of a full-dose anti-inflammatory tablet brought dramatic, near-complete relief within two days. The buttock pain sometimes shifts from one side to the other.
Clue 3
On examination, forward bending of his lower spine is strikingly limited: when the skin over the lower back is marked and he bends forward, the marked segment lengthens by only 2 cm (5 cm or more is normal). Chest expansion measured at the nipple line is reduced to 2.5 cm. He also recalls a painful, red, light-sensitive left eye last year that an eye specialist treated with steroid drops.
Clue 4
Blood tests show a normal white-cell count but a C-reactive protein of 22 mg/L (elevated) and an ESR of 40 mm/hr. Rheumatoid factor and anti-CCP antibodies are negative and the serum urate is normal. HLA-B27 testing is positive. A plain pelvic radiograph shows both sacroiliac joints with erosions, subchondral sclerosis, and joint-space narrowing, symmetrically on the two sides.
Diagnosis
Ankylosing Spondylitis
Why the diagnosis fits
This young man has chronic inflammatory back pain: insidious onset before age 40, night pain that wakes him in the second half of the night, more than an hour of morning stiffness, improvement with activity and NSAIDs rather than rest, and alternating buttock pain that localizes to the sacroiliac joints. Reduced lumbar flexion and chest expansion, a prior episode of acute anterior uveitis, HLA-B27 positivity, elevated CRP/ESR, and bilateral symmetric sacroiliitis progressing to thin marginal vertical syndesmophytes with vertebral squaring and fusion confirm axial spondyloarthritis of the classic radiographic type. The key discriminator from diffuse idiopathic skeletal hyperostosis is true inflammatory sacroiliitis with raised inflammatory markers (DISH spares the SI joints, produces flowing non-marginal ossification, and leaves CRP/ESR normal), while mechanical/degenerative back pain is excluded because it improves with rest, worsens with activity, and never bridges the spine.
Educational reference
The explanation and decisive case findings were checked against the source below.
Open the clinical referenceFor education and entertainment only. This fictional case is not medical advice and does not replace supervised clinical training, diagnosis, or treatment.