A Chiari II malformation is defined by which finding?

• A. Posterior cerebellum herniated downward through the foramen magnum with brainstem structures displaced caudally
• B. CSF release from the 4th ventricle obstructed
• C. Upward herniation of the temporal lobes
• D. Absence of the fourth ventricle

Answer: (A)

Chiari II malformation is defined by downward herniation of the hindbrain through the foramen magnum, with brainstem structures displaced caudally. This posterior fossa crowding pulls the cerebellar vermis and brainstem downward and is almost always seen in conjunction with open neural tube defects like myelomeningocele. The other findings described—upward herniation of temporal lobes, absence of the fourth ventricle, or CSF flow issues at the fourth ventricle—do not define Chiari II and reflect different processes.