A 52-year-old female with a 28-year history of schizophrenia, currently stable on risperidone 4 mg daily, is referred for cognitive evaluation by her group home staff who report progressive difficulty with meal preparation, medication management, and handling finances over the past two years. She was previously able to manage these tasks independently. Her MMSE score is 22/30, with deficits primarily in attention, recall, and visuospatial construction. She has no focal neurological deficits, and her MRI brain shows diffuse cortical atrophy without focal lesions or white matter changes. TSH, B12, folate, RPR, and metabolic panel are within normal limits. Her positive symptoms have been well controlled for the past decade, and she denies depressive symptoms. Which assessment approach most accurately distinguishes schizophrenia-related cognitive decline from an emerging neurodegenerative process in this patient?