**Core Concept**
Medulloblastoma is a highly malignant, metastatic primary CNS tumor that originates in the cerebellum or posterior fossa of children. It is known for its propensity to spread through the cerebrospinal fluid (CSF) and seed in other parts of the CNS.

**Why the Correct Answer is Right**
Medulloblastoma frequently metastasizes outside the brain via the CSF pathway, most commonly to the spinal cord, followed by the cranial nerves, and other parts of the CNS. This phenomenon occurs due to the tumor cells' ability to invade the leptomeninges and spread along the CSF spaces. The most common site of extracranial metastasis is the spinal cord, which can lead to symptoms such as back pain, weakness, and paralysis.

**Why Each Wrong Option is Incorrect**
**Option A:** Ependymoma is a type of CNS tumor that arises from the ependymal cells lining the ventricles and central canal of the spinal cord. While it can metastasize within the CNS, it is less likely to spread outside the brain compared to medulloblastoma.
**Option B:** Astrocytomas are the most common type of glioma in children, but they rarely metastasize outside the brain. When they do, it is usually within the CNS.
**Option C:** Craniopharyngioma is a type of brain tumor that typically occurs in children, but it is not known for metastasizing outside the brain.

**Clinical Pearl / High-Yield Fact**
Medulloblastoma is the most common malignant CNS tumor in children and is characterized by its aggressive behavior, including rapid growth and early metastasis. Recognizing the propensity of medulloblastoma to spread outside the brain is crucial for accurate diagnosis and effective treatment planning.

**Correct Answer:** A. Medulloblastoma.