A 4-year-old presents with cerebellar ataxia, nystagmus, and a posterior fossa mass on MRI. Histology shows pseudorosettes (Homer-Wright rosettes) and small round blue cells with high N:C ratio. The most likely diagnosis is:

• A Juvenile pilocytic astrocytoma
• B Ependymoma
• C Choroid plexus papilloma
• D Medulloblastoma ✓

Explanation

Medulloblastoma is the most common malignant brain tumor in children, arising in the cerebellar vermis. Homer-Wright rosettes (neuroblastic rosettes with neurofibrils in the center) and small round blue cells with scant cytoplasm are characteristic. Pilocytic astrocytoma is cystic with a mural nodule and shows biphasic pattern with Rosenthal fibers; ependymoma shows perivascular pseudorosettes with true rosettes; choroid plexus papilloma arises from choroid plexus.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.