The clinical features described – hypotonia, single palmar crease (simian crease), epicanthic folds, and a flattened facial profile – are classic signs of Down syndrome. Down syndrome is most commonly caused by Trisomy 21, meaning there are three copies of chromosome 21 instead of the usual two. Monosomy X (Turner syndrome) presents with different features like short stature, webbed neck, and primary amenorrhea in females. 47, XXY (Klinefelter syndrome) affects males, causing hypogonadism and gynecomastia. Deletion on chromosome 5p causes Cri-du-chat syndrome, characterized by a distinctive cat-like cry and microcephaly.
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