A 14-year-old boy has a painful mid-shaft tibial lesion. X-ray shows an 'onion skin' periosteal reaction. Biopsy reveals sheets of small round blue cells with nuclear moulding around vascular channels ('pseudo-rosettes'). The specific chromosomal translocation most commonly underlying this tumor is:

• A t(X;18)(p11;q11) resulting in SYT-SSX fusion gene
• B t(9;22)(q34;q11) resulting in BCR-ABL fusion gene
• C t(11;22)(q24;q12) resulting in EWS-FLI1 fusion gene ✓
• D t(12;16)(q13;p11) resulting in FUS-DDIT3 fusion gene

Explanation

The clinical and histological description — small round blue cells, onion skin periosteal reaction in a teenager's diaphysis — is classic Ewing sarcoma. The pathognomonic genetic aberration is t(11;22)(q24;q12) producing the EWS-FLI1 fusion oncogene, present in ~85% of Ewing sarcomas; it acts as an aberrant transcription factor driving tumour proliferation. t(X;18) is synovial sarcoma; t(9;22) BCR-ABL is CML; t(12;16) FUS-DDIT3 is myxoid liposarcoma. Recognition of this translocation is clinically important for molecular diagnosis and targeted therapy development.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.