Benign Paroxysmal Positional Vertigo (BPPV): This condition is common in the elderly and can be triggered by changes in head position, leading to brief but intense episodes of dizziness. Its prevalence and the age of the patient make it a likely candidate.

Orthostatic Hypotension: A common condition in the elderly due to decreased blood volume, medication effects, or autonomic dysfunction, leading to dizziness upon standing.
Vestibular Neuritis or Labyrinthitis: Inflammation of the inner ear can cause dizziness, and while not as common as BPPV, it's still a plausible diagnosis given the age and new onset of symptoms.
Medication Side Effects: Many medications can cause dizziness as a side effect, and elderly patients often have multiple prescriptions, increasing the risk.

Stroke or Transient Ischemic Attack (TIA): Although less likely, these conditions can present with dizziness and are medical emergencies. Missing them could be catastrophic.
Cardiac Arrhythmias: Conditions like atrial fibrillation can cause dizziness due to reduced cardiac output and are critical to diagnose due to their potential for severe complications.
Hypoglycemia: Especially in diabetic patients, hypoglycemia can cause dizziness and is easily treatable but dangerous if missed.

Acoustic Neuroma: A benign tumor on the nerve connecting the inner ear to the brain, which can cause dizziness among other symptoms, but is much less common.
Meniere's Disease: A disorder of the inner ear that affects balance and hearing, causing episodes of dizziness, but its onset at 90 years old would be unusual.
Multiple System Atrophy (MSA): A rare neurodegenerative disorder that can cause dizziness due to autonomic dysfunction, but is less likely given its rarity and the presence of other more prominent symptoms.