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
“Tectal beaking” is MRI feature of: 
Select and explain. 
A. Dandy Walker malformation, B. Arnold-Chiari malformation, C. Aqueductal stenosis, D. Third ventricular tumor
B: Arnold-Chiari malformation
Answer: B: Arnold-Chiari malformation 
Explanation: Arnold Chiari I malformation
Chiari I malformation (cerebellar ectopia and seen in adulthood.
Definition: - Descent of otherwise normal cerebellar hemispheres below the foramen magnum, usually involving tonsils.
Features: Peg like, pointed tonsils displaced into upper cervical canal
 0–10 years à 6 mm at least
10–30 years à 5 mm
 30–40 years à 4 mm
Associated anomalies: Syringomyelia (30–60%), hydrocephalus (20–25%), basilar invaginations (25–50%), Klippel-Feil syndrome (5–10%) and atlanto-occipital assimilation (1–5%)
Chiari II malformation (childhood).
Definition: 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.
Features: