Frontal Temporal Dementia (FTD): This is a group of brain disorders caused by cellular degeneration that affects the frontal and temporal lobes of the brain. The acute onset of personality changes, such as apathy, disinhibition, or compulsive behaviors, is a hallmark symptom of the behavioral variant of FTD, making it a strong candidate for the single most likely diagnosis.

Major Depressive Disorder: Acute onset of depressive symptoms can sometimes manifest as significant personality changes, including apathy, social withdrawal, or irritability.
Bipolar Disorder: Manic or hypomanic episodes can lead to sudden changes in personality, such as increased impulsivity or decreased inhibitions.
Substance-Induced Personality Change: The use of certain substances, including drugs and alcohol, can lead to acute changes in personality, especially if there's a sudden increase in use or a change in the type of substance used.
Traumatic Brain Injury: Depending on the area of the brain affected, a traumatic brain injury can result in personality changes that seem acute in onset.

Neurosyphilis: This is a rare but potentially treatable condition that can cause personality changes, among other neurological symptoms. Missing this diagnosis could lead to severe and irreversible neurological damage.
Brain Tumors: Tumors in certain areas of the brain, especially the frontal lobe, can cause sudden personality changes. Early detection is crucial for effective treatment.
Creutzfeldt-Jakob Disease: A rare, degenerative, and fatal brain disorder that can cause rapid changes in personality, along with other neurological symptoms. Although rare, its rapid progression and fatal outcome make it a "do not miss" diagnosis.
Vascular Dementia: Sudden onset of personality changes could be indicative of vascular dementia, especially if there are other signs of cognitive decline. This condition is critical to identify due to its implications for stroke prevention and management.

Huntington's Disease: An inherited disorder that causes brain cells to die, leading to problems with movement, emotion, and thinking. Personality changes can be an early symptom, but the disease's overall rarity and specific genetic nature make it less likely.
Wilson's Disease: A genetic disorder that leads to copper accumulation in the brain and liver. It can cause neurological symptoms, including personality changes, but it is very rare and typically presents in younger individuals.
Prion Diseases (other than Creutzfeldt-Jakob Disease): Other prion diseases, like fatal familial insomnia, can cause personality changes but are extremely rare and usually have distinctive additional symptoms.