On plain X-ray, a 35-year-old man with known psoriasis shows 'pencil-in-cup' deformity at the DIP joints of the hands and feet. This pattern is diagnostic of which arthropathy?
- A Rheumatoid arthritis — pannus erosions
- B Psoriatic arthritis — mutilans variant with 'cup and pencil' deformity ✓
- C Gout — tophi with overhanging edges
- D Reactive arthritis (Reiter's) — periostitis
Explanation
The 'pencil-in-cup' deformity is characteristic of psoriatic arthritis, specifically the arthritis mutilans (most destructive) variant. One bone end erodes to a sharp pencil point while the opposing bone end cups around it, creating the radiographic appearance. This occurs at DIP joints preferentially (unlike RA which spares DIPs), and is associated with the 'ivory phalanx' (terminal phalanx sclerosis) and DIP joint involvement. Gout shows asymmetric 'rat bite' erosions with overhanging edges ('Martel sign'). RA shows symmetrical periarticular erosions at MCPs/PIPs. Reactive arthritis causes periostitis, sacroiliitis, and Achilles enthesitis.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.