This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Unknown
[{'content': '“Tectal beaking” is MRI feature of: \nSelect and explain. \nA. Dandy Walker malformation, B. Arnold-Chiari malformation, C. Aqueductal stenosis, D. Third ventricular tumor', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Arnold Chiari I malformation\nChiari I malformation (cerebellar ectopia and seen in adulthood.\nDefinition: - Descent of otherwise normal cerebellar hemispheres below the foramen magnum, usually involving tonsils.\nFeatures: Peg like, pointed tonsils displaced into upper cervical canal\n\uf0a1 0–10 years à 6 mm at least\n\uf0a110–30 years à 5 mm\n\uf0a1 30–40 years à 4 mm\nAssociated anomalies: Syringomyelia (30–60%), hydrocephalus (20–25%), basilar invaginations (25–50%), Klippel-Feil syndrome (5–10%) and atlanto-occipital assimilation (1–5%)\nChiari II malformation (childhood).\nDefinition: Descent of dysplastic cerebellar hemisphere below the foramen magnum, usually involving inferior vermis, which is everted instead being inverted, causing 4th ventricle to reduce to a coronal cleft and medulla oblongata to elongate and kink.\nFeatures:', 'role': 'assistant'}]