On X-ray of the knee, a soap-bubble lytic lesion eccentrically placed in the epiphysis and extending to the subchondral bone of the distal femur is seen in a 25-year-old. MRI shows fluid-fluid levels. The MOST likely diagnosis is:

• A Aneurysmal bone cyst ✓
• B Giant cell tumor (GCT)
• C Chondroblastoma
• D Clear cell chondrosarcoma

Explanation

Fluid-fluid levels on MRI (due to sedimented blood products of different ages) are the hallmark of aneurysmal bone cyst (ABC). ABC can occur in the epiphysis, and when it appears in this region of a young adult with an eccentric lesion, it can mimic GCT. GCT does not typically show fluid-fluid levels. Chondroblastoma occurs in the epiphysis in skeletally immature patients but shows chondroid matrix calcification. Clear cell chondrosarcoma is rare and has characteristic cartilaginous matrix. ABC is the correct answer because of the fluid-fluid levels.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.