A neonate born at term presents with myelomeningocele at L3–L4 and a Chiari II malformation on MRI. Which embryological event is the IMMEDIATE cause of the hindbrain herniation seen in Chiari II?

• A Premature closure of the anterior fontanelle restricts cranial vault expansion
• B Overgrowth of the rhombic lip produces excess neural tissue
• C Failure of development of the alar plate at the cervical level
• D Failure of closure of the posterior neuropore leads to loss of cerebrospinal fluid turgidity and a small posterior fossa ✓

Explanation

In Chiari II malformation the open spinal defect (failure of posterior neuropore closure) allows CSF to leak continuously in utero, reducing intraventricular pressure. This impairs the normal CSF-driven distension that shapes the posterior cranial fossa; the resulting small posterior fossa cannot accommodate the hindbrain, forcing the cerebellar vermis, medulla, and fourth ventricle to herniate through the foramen magnum. The other options describe distinct embryological events not mechanistically linked to Chiari II.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.