A rare case of anti-NMDA receptor encephalitis presenting with subacute neuropsychiatric symptoms in an older patient.

Anti-NMDA receptor encephalitis is a rare autoimmune condition that often presents with delirium and seizures. While classically associated with younger women and ovarian teratomas, it can occur across a broader demographic and mimic a psychotic disorder or metabolic conditions. This case describes a 59-year-old woman with a history of fibromyalgia and depressive disorder who presented with subacute delirium, behavioral disturbances, and incontinence. Initial evaluations were unrevealing, and early diagnostic considerations attributed her presentation to a psychotic disorder. However, further workup, including cerebrospinal fluid antibody testing, confirmed anti-NMDA receptor encephalitis. This case underscores the diagnostic challenges posed by autoimmune encephalitis and highlights the importance of maintaining a broad differential. Anchoring bias and premature diagnostic closure can delay life-saving treatment; clinicians must remain vigilant when evaluating unexplained subacute neuropsychiatric decline.