An 82-year-old woman admitted to the medical ward for pneumonia develops acute confusion, agitation, and visual hallucinations over 12 hours. Her daughter reports the patient was cognitively intact 2 days ago. On examination, her consciousness level fluctuates, and she is inattentive. Blood cultures and chest X-ray confirm bacterial pneumonia. Which feature most reliably distinguishes this acute presentation from a chronic neurodegenerative dementia?