This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetics, Subject name: Pathology
Most common chromosomal defect is: 
Select and explain. 
A. Down's, B. Turner, C. Edward, D. Patau
A: Down's
Answer: A: Down's 
Explanation: Ans: a (Downs) Ref: Robbins, 7th ed, p. 175Down's syndrome is the most common of the chromosome disorders and a major cause of mental retardation.Cytogenetic abnormalityNon disjunction (MC) = 95%Translocation (robertsonian) = 4 %Mosaicism = 1%Increased maternal age is the main risk factor .Translocation carriers carry high recurrence risk.Clinical features:Cardiac - endocardial cushion defect (MC)HypotoniaGIT - duodenal atresia, TEF, IHPS, Hirschprung disease, annular pancreasMental retardationShort statureMusculoskeletal system - atlantoaxial dislocationMicrocephaly/ brachycephalyRespiratory - recurrent aspiration pneumoniaOpen AFEndocrine - primary gonadal defect, hypothyroidismOblique palpebral fissureHaematopoietic - increased ALL and AMLBmshfield spot. But AML has a betterprognosis than normal individualsDepressed nasal bridgesLow set earsCNS alzhiemers in patients older than 40 yearsShort neck Simian crease,clinodactyly Sandal gap between 1st and 2nd toes Kennedy crease