Final answer:
Post-rotary nystagmus that lasts less than 7 seconds is indicative of normal vestibular function. The correct answer is option a).
Step-by-step explanation:
Post-rotary nystagmus (PRN) lasting less than 7 seconds typically indicates normal vestibular function. Nystagmus is an involuntary eye movement that is often stimulated by rotation of the body, which in turn stimulates the vestibular system in the inner ear. The duration of nystagmus after rotation has stopped can reflect the health of this system.
A short duration of nystagmus, such as less than 7 seconds, is commonly considered normal because it suggests the vestibular system is effectively bringing the eye movements under control promptly, indicating its ability to stabilize the visual field and maintain balance is intact.
In contrast, prolonged nystagmus might suggest vestibular dysfunction, which can be associated with conditions like Ménière's disease or damage to the brain stem affecting eye movement nuclei or their neural pathways.