MRI of a young adult with occipital headaches and downbeat nystagmus shows the cerebellar tonsils lying 9 mm below a line drawn from the basion to the opisthion across the foramen magnum. What is this reference line called?

• A Chamberlain's line
• B McGregor's line
• C McRae's line ✓
• D Wackenheim's line

Explanation

McRae's line joins the basion, the anterior margin of the foramen magnum, to the opisthion, its posterior margin, and therefore measures tonsillar descent directly across the foramen magnum. Tonsils more than 5 mm below this line define a Chiari I malformation. Chamberlain's line runs from the hard palate to the opisthion and McGregor's line from the hard palate to the lowest point of the occiput; both assess basilar invagination rather than tonsillar position.

Reference: Grainger and Allison's Diagnostic Radiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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