This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.N.S, Subject name: Medicine
[{'content': "Alzeihmer's disease is associated with \nSelect and explain. \nA. Chr 21, B. Chr 19, C. Chr.21, D. Chr.22", 'role': 'user'}, {'content': "Answer: A \nExplanation: Patients with trisomy 21 have 3 copies of the gene for amyloid precursor protein and develop neurological characteristics of Alzheimer's disease by midlife. Alzheimer's Disease in People with Down Syndrome. Many, but not all, people with Down syndrome develop Alzheimer's disease when they get older. People with Down syndrome are born with an extra copy of chromosome 21, which carries the APP gene. This gene produces a specific protein called amyloid precursor protein (APP) .which leads to development of Alzheimer's Ref Harrison20th edition pg 2756", 'role': 'assistant'}]