This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Growth and development, Subject name: Pediatrics
Down syndrome most commonly occurs due to ____ 
Select and explain. 
A. Reciprocal translocation, B. Non-disjunction in maternal meiosis, C. Mosaicism, D. Non-disjunction in paternal meiosis
B: Non-disjunction in maternal meiosis
Answer: B: Non-disjunction in maternal meiosis 
Explanation: Down&;s syndrome occurs due to nondisjunction in maternal meiosis. Down&;s syndrome: Most common chromosomal disorder Incidence: 1/800 live bihs Mechanism: Trisomy 21 (meiotic maternal nondisjunction) (95%) Robesonian translocation: 4% Mosaic pattern: 1% Non-disjunction points to the failure of the homologous chromosome to seperate during the meiotic division. Ref: Nelson textbook of pediatrics 21st edition Pgno: 658