Option D is correct because purely vertical nystagmus (either upbeating or downbeating) or purely torsional nystagmus is always a sign of a central cause and does not result from a peripheral lesion | Option A is incorrect because horizontal-torsional nystagmus is the typical pattern of acute peripheral vestibular lesions | Option B is incorrect because suppression of nystagmus by visual fixation is a classic feature of peripheral lesions, whereas central nystagmus is not suppressed by fixation | Option C is incorrect because unidirectional nystagmus (following Alexander's law) is a typical feature of peripheral lesions, whereas direction-changing nystagmus indicates a central lesion | Consult a neurologist or vestibular specialist immediately if nystagmus is purely vertical or direction-changing, as this indicates a brainstem or cerebellar lesion and may require urgent neuroimaging | The single variable that changes the answer: presence of accompanying central neurological symptoms (diplopia, paresis, ataxia) converts the case to a neurological emergency requiring immediate MRI | Consultant Pearl: In peripheral vestibular lesions, nystagmus is always horizontal-torsional, suppressed by visual fixation, and follows Alexander's law; any deviation from this pattern indicates a central lesion | Sources: vertigo.pdf — p. 3; CUMMINGS OTOLARYNGOLOGY–HEAD AND NECK 7th Ed p1.pdf — p. 1023; Scott Browns Otorhinolaryngology 8th edition volume 2 .pdf — p. 824