Question:
Identify the condition shown in the CT scan image below.
Identify the condition shown in the CT scan image below.
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Vermian agenesis + cystic 4th ventricle + enlarged posterior fossa.
Updated On: Jun 24, 2026
- Dandy-Walker malformation
- Cerebellar vermis hypoplasia
- Mega cisterna magna
- None
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The Correct Option is A
Solution and Explanation
Step 1: The CT image shows the classic posterior fossa abnormality of Dandy-Walker malformation. This is a developmental anomaly of the cerebellum and the fourth ventricle.
Step 2: The diagnostic triad is hypoplasia or agenesis of the cerebellar vermis, cystic dilatation of the fourth ventricle that fills an enlarged posterior fossa, and upward displacement of the tentorium and torcula. The large posterior fossa cyst continuous with the fourth ventricle, together with the splayed cerebellar hemispheres, identifies Dandy-Walker malformation. Hence option 1 is correct.
Step 3: Why the others are wrong. Isolated cerebellar vermis hypoplasia lacks the large communicating fourth-ventricle cyst and the enlarged posterior fossa. Mega cisterna magna is an enlarged cisterna magna with an intact vermis and normal fourth ventricle. Therefore the full picture shown is Dandy-Walker malformation, not these milder variants.
Step 2: The diagnostic triad is hypoplasia or agenesis of the cerebellar vermis, cystic dilatation of the fourth ventricle that fills an enlarged posterior fossa, and upward displacement of the tentorium and torcula. The large posterior fossa cyst continuous with the fourth ventricle, together with the splayed cerebellar hemispheres, identifies Dandy-Walker malformation. Hence option 1 is correct.
Step 3: Why the others are wrong. Isolated cerebellar vermis hypoplasia lacks the large communicating fourth-ventricle cyst and the enlarged posterior fossa. Mega cisterna magna is an enlarged cisterna magna with an intact vermis and normal fourth ventricle. Therefore the full picture shown is Dandy-Walker malformation, not these milder variants.
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