A 28-year-old woman presents with a 3-week history of painful monocular vision loss with a central scotoma and a relative afferent pupillary defect (RAPD). Brain MRI shows multiple T2/FLAIR hyperintense lesions, including the structure marked **A** in the diagram—ovoid lesions with long axis perpendicular to the lateral ventricles. Cerebrospinal fluid analysis reveals oligoclonal bands. Based on the characteristic MRI findings and clinical presentation, which of the following best describes the pathophysiology of the lesions marked **A**?
A. Infratentorial demyelination in the brainstem and cerebellar white matter tracts
B. Perivenular inflammation along medullary veins causing demyelination perpendicular to ventricular walls
C. Inflammation at the gray-white matter junction without specific orientation to ventricular anatomy
D. Subpial cortical inflammation with involvement of gray matter at the cortical surface
Explanation
Why "Perivenular inflammation along medullary veins causing demyelination perpendicular to ventricular walls" is right
The structure marked A represents Dawson fingers—the pathognomonic periventricular ovoid lesions of MS. The McDonald Criteria 2017 and AAN MS Guidelines emphasize that these lesions are characterized by their perpendicular orientation to the ventricles, reflecting perivenular inflammation along medullary veins. This vascular-centric pattern of demyelination is a hallmark of MS and is critical for establishing dissemination in space (DIS) on MRI. The patient's clinical presentation (optic neuritis with RAPD, oligoclonal bands) combined with these characteristic MRI findings fulfills the 2017 McDonald Criteria for MS diagnosis.
Why each distractor is wrong
Subpial cortical inflammation with involvement of gray matter at the cortical surface: This describes juxtacortical lesions (marked C in the diagram), not periventricular lesions. While juxtacortical lesions are part of MS pathology, they do not have the perpendicular orientation to ventricles that defines Dawson fingers.
Inflammation at the gray-white matter junction without specific orientation to ventricular anatomy: This is a non-specific description that fails to capture the defining feature of periventricular lesions—their perpendicular alignment to the ventricles and association with medullary veins. This vague characterization would not distinguish MS from other demyelinating conditions.
Infratentorial demyelination in the brainstem and cerebellar white matter tracts: While infratentorial lesions (brainstem, cerebellum) are indeed characteristic of MS and contribute to DIS, they are a separate category from periventricular lesions and do not explain the specific morphology of Dawson fingers.
High-YieldNEET PG
Periventricular ovoid lesions perpendicular to ventricles (Dawson fingers) = perivenular inflammation along medullary veins = hallmark of MS and key criterion for DIS in McDonald 2017.
AAN MS Guidelines + McDonald Criteria 2017
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