A 13-year-old boy presents with chronic knee pain. X-ray shows a well-defined lytic lesion in the proximal tibial epiphysis with stippled calcification. Histology shows rounded cells with grooved nuclei surrounded by multinucleated giant cells, with pericellular 'chicken wire' calcification. The immunohistochemical marker expected to be positive is:
- A CD99
- B S-100 protein ✓
- C Brachyury
- D Epithelial membrane antigen
Correct answer: B. S-100 protein
Explanation
Chondroblastoma occurs in the epiphyses of adolescents before epiphyseal closure, shows chicken wire calcification, and its chondroid tumour cells are positive for S-100 protein. CD99 positivity belongs to Ewing sarcoma and other small round blue cell tumours, which is the key distractor here because both occur in young patients around the knee. Brachyury marks chordoma, and EMA positivity suggests meningioma or epithelial tumours.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.