This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Specific Cytogenetic Disorders, Subject name: Pathology
Down's syndrome is: 
Select and explain. 
A. Trisomy 21, B. Trisomy 18, C. Trisomy 13, D. 45,X
A: Trisomy 21
Answer: A: Trisomy 21 
Explanation: Symptoms of down's syndrome (Trisomy 21): Mental retardation Epicanthic folds and flat facial profile Simian's crease Umblical hernia Hypotonia Gap between first and second toe Congenital hea defects Trisomy 18 (Edwards syndrome) symptoms: Prominent occiput Mental retardation Low set ears Congenital hea defects Rocker-bottom feet Trisomy 13 (Patau syndrome) symptoms: Microphthalmia Microcephaly and mental retardation Polydactyly Cleft lip and palate Renal defects Turner syndrome (45,X) include: Sho stature Coarctation of aoa Cubitus valgus Streak ovaries, infeility, amenorrhea Peripheral lymphedema at bih Low posterior hairline Webbing of neck Broad chest and widely spaced nipples