Which statement best describes the relationship between Chiari II malformation and hydrocephalus?

• A. Chiari II malformation always causes hydrocephalus
• B. Chiari II malformation has no effect on CSF dynamics
• C. Chiari II malformation can contribute to hydrocephalus by obstructing CSF flow at the fourth ventricle
• D. Chiari II malformation only occurs in the absence of hydrocephalus

Answer: (C)

Chiari II malformation can distort hindbrain structures enough to disrupt CSF flow, particularly around the fourth ventricle and its outlets. This impaired CSF circulation can lead to hydrocephalus because the usual pathways for CSF exit and absorption are obstructed or altered. It’s not universal—some individuals with Chiari II don’t develop hydrocephalus—but when hydrocephalus occurs, obstruction at the fourth ventricle/outflow pathways is a plausible contributing mechanism. The other statements aren’t as accurate because the malformation does affect CSF dynamics, hydrocephalus is not present in every case, and Chiari II does not occur only in the absence of hydrocephalus.