Schizophrenia: This is a primary consideration due to the episodic psychosis, which is a hallmark symptom. Cognitive decline can also be a feature, especially in untreated or severe cases.

Bipolar Disorder with Psychotic Features: Episodes of psychosis can occur during manic or depressive phases, and cognitive decline may be observed, particularly if the condition is not well-managed.
Substance-Induced Psychotic Disorder: Certain substances can induce psychosis and cognitive impairments, and the episodic nature could be related to substance use patterns.
Major Depressive Disorder with Psychotic Features: Though less common, severe depression can include psychotic episodes, and significant cognitive decline can occur, especially in older adults.

Neurosyphilis: A potentially treatable condition that can cause psychosis and cognitive decline. Missing this diagnosis could lead to irreversible neurological damage.
HIV-Associated Neurocognitive Disorder (HAND): Given the potential for severe consequences if untreated, considering HIV infection is crucial, especially if there are risk factors or other suggestive symptoms.
Lyme Disease: Neuroborreliosis, a manifestation of Lyme disease, can cause psychiatric symptoms, including psychosis, and cognitive issues, making it a critical diagnosis not to miss.
Vitamin B12 Deficiency: This condition can lead to neurological and psychiatric symptoms, including psychosis and cognitive decline, and is easily treatable with supplementation.

Prion Diseases (e.g., Creutzfeldt-Jakob Disease): These are rare, fatal brain disorders that can cause rapid cognitive decline and psychiatric symptoms, including psychosis.
Wilson's Disease: A genetic disorder that leads to copper accumulation in the brain, causing neurological and psychiatric symptoms, including psychosis and cognitive decline.
Hashimoto's Encephalopathy: A rare condition associated with Hashimoto's thyroiditis, characterized by cognitive decline and psychiatric symptoms, including psychosis.