A 20-week anomaly scan shows a single large midline ventricle communicating with a dorsal cyst, completely fused thalami, absence of the cavum septi pellucidi and corpus callosum, and hypotelorism with a midline facial cleft. The most likely diagnosis is:
- A Severe aqueductal stenosis
- B Alobar holoprosencephaly ✓
- C Lissencephaly
- D Schizencephaly
Correct answer: B. Alobar holoprosencephaly
Explanation
Failure of cleavage of the prosencephalon produces holoprosencephaly, and the alobar form shows a single monoventricle often with a dorsal cyst, fused thalami, absent midline structures, and characteristic facial anomalies including hypotelorism, cyclopia, proboscis, or midline cleft lip and palate. The strongest chromosomal association is trisomy 13. Aqueductal stenosis causes dilated lateral and third ventricles with preserved midline anatomy, and schizencephaly shows a cleft lined by grey matter connecting ventricle to cortex.
Reference: Callen's Ultrasonography in Obstetrics and Gynecology, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.