Vestibular Neuritis: This condition is characterized by inflammation of the vestibular nerve, leading to sudden onset of vertigo, nausea, and gait ataxia. The negative MRI helps to rule out central causes, making vestibular neuritis a plausible diagnosis given its relatively high incidence and the match between symptoms and presentation.

Benign Paroxysmal Positional Vertigo (BPPV): Although BPPV typically presents with brief episodes of vertigo triggered by specific head movements, some cases can have a more prolonged course. The absence of central nervous system findings on MRI and the presence of gait ataxia could still align with BPPV, especially if the patient experiences vertigo with positional changes.
Labyrinthitis: An inner ear disorder characterized by inflammation of the labyrinth, which can cause vertigo, hearing loss, and gait ataxia. The symptoms presented could fit labyrinthitis, especially if there's an associated viral or bacterial infection.
Orthostatic Hypotension: While primarily causing dizziness upon standing, orthostatic hypotension can also lead to nausea and ataxia due to decreased blood flow to the brain. This condition is common in the elderly and could be considered, especially if symptoms worsen with standing.

Posterior Circulation Stroke: Despite a negative MRI, it's crucial to consider a stroke, particularly in the posterior circulation, which can sometimes be missed on initial imaging. Symptoms like acute onset dizziness, nausea, and gait ataxia can be indicative of a stroke in the cerebellum or brainstem.
Multiple Sclerosis: Although less likely given the patient's age and acute presentation, multiple sclerosis (MS) can cause a wide range of neurological symptoms, including ataxia and vertigo. A high index of suspicion is necessary to not miss this potentially treatable condition.
Wernicke's Encephalopathy: This condition, caused by thiamine deficiency, can present with ataxia, ophthalmoplegia, and confusion. Given its potentially fatal outcome if untreated, it's essential to consider, especially in malnourished or alcoholic patients.

Acoustic Neuroma: A benign tumor on the vestibular nerve can cause gradual onset of vertigo, hearing loss, and ataxia. While the acute onset in this case makes it less likely, it's a consideration if symptoms persist or progress.
Chiari Malformation: A structural defect in the cerebellum that can cause symptoms similar to those presented, especially with coughing, sneezing, or straining. However, it's relatively rare and often diagnosed in younger patients.
Superior Canal Dehiscence: A rare condition where there's an abnormal opening in the bone overlying the superior canal, leading to vertigo and hearing symptoms in response to sound or pressure changes. The presentation can be variable, making it a rare but possible consideration.