A 16-year-old male presents with fever, weight loss, and an aggressive lytic lesion of the femoral diaphysis with a classic 'onion peel' periosteal reaction. Biopsy shows small round blue cells arranged in sheets with PAS-positive glycogen granules. Which chromosomal translocation is pathognomonic?
- A t(X;18)(p11;q11) — SS18-SSX fusion
- B t(2;13)(q35;q14) — PAX3-FOXO1 fusion
- C t(11;22)(q24;q12) — EWS-FLI1 fusion ✓
- D t(12;16)(q13;p11) — FUS-DDIT3 fusion
Correct answer: C. t(11;22)(q24;q12) — EWS-FLI1 fusion
Explanation
Ewing sarcoma is characterised by the t(11;22)(q24;q12) translocation in ~85% of cases, creating the EWS-FLI1 fusion oncogene that drives tumour growth. Small round blue cells with PAS-positive glycogen, diaphyseal involvement, onion-skin periosteal reaction, and systemic symptoms are classic features. t(X;18) is synovial sarcoma; t(2;13) is alveolar rhabdomyosarcoma; t(12;16) is myxoid liposarcoma. These small round cell tumours must be distinguished because treatment differs significantly.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.