A 20-week anomaly scan shows the fetal cerebellum displaced posteriorly with an S-shaped or banana sign, and the frontal bones show inward scalloping (lemon sign). These signs are most characteristic of:
- A Dandy-Walker malformation
- B Agenesis of the corpus callosum
- C Vein of Galen malformation
- D Open neural tube defect (spina bifida aperta) with Arnold-Chiari II malformation ✓
Explanation
The banana sign (curved, banana-shaped cerebellum displaced posteriorly into the cisterna magna) and lemon sign (frontal bone scalloping from negative intracranial pressure) are the classical second-trimester ultrasound signs of Arnold-Chiari II malformation, which invariably accompanies open spina bifida. The Chiari II downward herniation obliterates the cisterna magna and distorts the cerebellum. Dandy-Walker malformation shows posterior fossa cyst with vermian agenesis and splaying of the cerebellar hemispheres. ACC shows parallel lateral ventricles and absent cavum septi pellucidi.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.