A 2-year-old boy is brought to the physician by his parents, who complain that their son continually loses his balance. They also report that his speech seems more slurred. Physical examination confirms the truncal ataxia and wide-based gait. The child appears lethargic, and there is bobbing of the head while he is sitting. Muscle tone is normal. A lumbar puncture is performed in the patient, and neoplastic cells are found. A suboccipital craniotomy is performed, and a tumor is resected. Microscopic examination of the surgical specimen would most likely reveal which of the following histologic patterns?

Medulloblastomas and, less commonly ependymomas and pineal parenchymal tumors, have a propensity to disseminate by way of the CSF throughout the CNS. Since medulloblastomas are close to the fourth ventricle, they may disseminate downstream into the spinal cord. The detection of neuronal and progenitor cell markers (e.g., synaptophysin or nestin) may be diagnostically informative in distinguishing these tumors from metastatic epithelial tumors, lymphomas, or other neuroectodermal malignancies. Medulloblastomas are characterized by cells with hyperchromatic, round-to-oval nuclei and scant cytoplasm, which often crowd together with no structural pattern. The neuroblastic character of the cells is occasionally expressed in rosette formation, a histologic feature of neuroblasts. The other choices do not describe midline, cerebellar tumors arising in children.Diagnosis: Medulloblastoma