A patient with psoriatic arthritis has developed a 'pencil-in-cup' deformity of the distal interphalangeal joint on radiograph. Which histopathological process is responsible?
- A Pannus formation causing marginal erosion and subchondral sclerosis
- B Calcium pyrophosphate deposition causing chondrocalcinosis
- C Proliferative enthesitis with osteolysis of the phalanx tip and reactive new bone at the base ✓
- D Urate crystal deposition causing erosions with overhanging edges
Correct answer: C. Proliferative enthesitis with osteolysis of the phalanx tip and reactive new bone at the base
Explanation
Psoriatic arthritis produces an enthesitis-driven process causing marked osteolysis of the distal phalanx (the 'pencil') which telescopes into the expanded, remodelled base of the proximal phalanx (the 'cup'), creating the pathognomonic pencil-in-cup appearance. This is distinct from RA pannus (marginal erosions with periarticular osteoporosis), gout (punched-out erosions with overhanging edges and no periarticular osteoporosis), and CPPD (chondrocalcinosis).
Reference: Maheshwari Essential Orthopaedics, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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