A Rare Double Aneuploidy Case (Down-Klinefelter).

Down's syndrome has its own dysmorphic findings and is accompanied by mental retardation and hypotonia. Klinefelter's syndrome is a syndrome caused by a numerical abnormality that affects male physical and cognitive development. This case reports a unique finding of 48,XXY, + 21 and a current literature review. A 4-month-old male patient presented with typical clinical features of Down's syndrome with hypothyroidism, atrial septal defect, ventricular septal defect, and patent ductus arteriosus without any phenotypic signs of Klinefelter's syndrome.