Persistent downbeat nystagmus observed in the primary position localizes classically to dysfunction of which structure?
- A Cerebellar flocculus at the cerebellopontine angle or cervicomedullary junction ✓
- B Vestibular nuclei of the pons alone
- C Medial geniculate body of the thalamus
- D Oculomotor nucleus complex of the midbrain
Correct answer: A. Cerebellar flocculus at the cerebellopontine angle or cervicomedullary junction
Explanation
Downbeat nystagmus arises from imbalance in the vertical vestibulo-ocular reflex pathways and localizes to the cerebellar flocculus or the cervicomedullary junction. Classic causes include Arnold-Chiari malformation, cerebellar degeneration, and drugs such as lithium or anticonvulsants. Lesions of the nodulus and uvula instead produce periodic alternating nystagmus through impaired velocity storage, which is the key contrast with option B's region.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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