This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Central Nervous System, Subject name: Pathology
Chromosomes involved in early onset Alzheimer's disease: 
Select and explain. 
A. 1,14,21, B. 1,13,22, C. 3, 14, 20, D. 3, 14, X
A: 1,14,21
Answer: A: 1,14,21 
Explanation: A i.e. 1,14,21Ref: Genetic Home ReferenceExplanation:Genes Involved in Alzheimers Disease:Early onset Alzehimer's disease involves three genes: APP-Ch 21.PSEN1 (Presenilin 1) - Ch 14.PSEN 2 (Presenilin 2) - Ch 1.Genes causing autosomal dominant AD* APP* PSEN1* PSEN 2All can cause early onset disease but PSEN2 is associated with a broad range of onset ages.Susceptibitity genes* Apolipoprotein E gene (APOE)- c2 allele: associated with decreased risk- s3 allele: neutral- e4 allele: high-risk* SORL1