This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
Which of the following is not a feature of 21st trisomy? 
Select and explain. 
A. Mental retardation, B. Duodenal atresia, C. Hypeonicity, D. CHD
C: Hypeonicity
Answer: C: Hypeonicity 
Explanation: Down's syndrome (21st trisomy) is the most common chromosomal disorder and a major cause of mental retardation. Children with Down's syndrome develop muscle hypotonia not hypeonia. Reference: Essential Paediatrics by O.P Ghai 6e page 592.