Frontotemporal Dementia (FTD): This condition is characterized by changes in personality, behavior, and language due to degeneration of the frontal and temporal lobes of the brain. The patient's symptoms of disinhibited behavior, forgetfulness, and delusions of grandiosity are consistent with the behavioral variant of FTD, which often presents in individuals under the age of 65.

Bipolar Disorder: The presence of delusions of grandiosity could suggest a manic episode, which is a hallmark of bipolar disorder. However, the progressive nature of the symptoms over 2-3 years and the presence of significant forgetfulness might lean more towards a neurodegenerative condition.
Schizophrenia: While schizophrenia can present with delusions and behavioral changes, the age of onset and the specific combination of symptoms (especially significant forgetfulness) are less typical for schizophrenia.
Traumatic Brain Injury (TBI): A history of TBI could lead to personality changes, memory issues, and even psychotic symptoms, depending on the area of the brain affected. However, there's no mention of such a history in the provided information.

Huntington's Disease: Although less common, Huntington's disease can present with psychiatric symptoms, including personality changes and delusions, along with cognitive decline. It's crucial to consider this diagnosis due to its significant implications for genetic counseling.
Prion Diseases (e.g., Creutzfeldt-Jakob Disease): These rare, fatal brain diseases can cause rapid cognitive decline, personality changes, and psychiatric symptoms. Given their fatal nature and the need for prompt diagnosis to prevent further transmission (in the case of iatrogenic transmission), they should not be missed.
Vitamin B12 Deficiency: While less likely to cause delusions of grandiosity, a severe vitamin B12 deficiency can lead to neurological and psychiatric symptoms, including cognitive impairment and personality changes. It's easily treatable and thus important not to miss.

Wilson's Disease: A genetic disorder that leads to copper accumulation in the brain and liver. It can cause psychiatric symptoms, cognitive decline, and neurological signs. The age of the patient and the specific symptoms could fit, but it's less common.
Neurosyphilis: A condition that occurs when syphilis infection spreads to the brain. It can cause a wide range of psychiatric and neurological symptoms. Given the availability of effective treatment, considering this diagnosis is important, especially if other causes are ruled out.