A 15-year-old girl presents with rapidly progressive pain and swelling of the upper arm. X-ray shows an eccentric, expansile lytic lesion in the proximal humerus metaphysis that has ballooned beyond the cortex with a thin eggshell rim. MRI shows multiple fluid-fluid levels within the lesion. The characteristic microscopic finding is:

• A Nests of epithelial cells in a fibrous stroma
• B Curvilinear trabeculae of woven bone in a fibrous stroma without osteoblastic rimming
• C Sheets of uniform small round blue cells with glycogen
• D Blood-filled spaces separated by fibrous septa containing giant cells and osteoid ✓

Explanation

Aneurysmal bone cyst consists of blood-filled cavernous spaces separated by connective tissue septa carrying fibroblasts, osteoclast-type giant cells, and reactive woven bone with osteoblastic rimming. It typically affects the metaphysis in the first two decades, expands the cortex with an eggshell rim, and shows fluid-fluid levels on MRI. Curvilinear woven bone without osteoblastic rimming describes fibrous dysplasia (option B), the main distractor.

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.