A 22-year-old primigravida notes absent fetal movement for 2 days. The fetus is delivered stillborn at 19 weeks' gestation. The macerated fetus shows marked hydrops fetalis and a large posterior cystic hygroma of the neck. At autopsy, internal anomalies include aortic coarctation and a horseshoe kidney. Which of the following karyotypes is most likely to be present in cells obtained from this fetus?

The findings listed are characteristic of Turner syndrome (monosomy X), which accounts for many first- trimester fetal losses. The hygroma is quite suggestive of this disorder. Fetuses with this finding are rarely live-born. Trisomy 18 can be marked by multiple anomalies, but overlapping fingers and a short neck are more typical features. Down syndrome (47, XX, +21) may be accompanied by a hygroma and hydrops, but ventricular septal defect is more frequent than coarctation, and horseshoe kidney is uncommon. The 47, XXY karyotype (Klinefelter syndrome) does not result in stillbirth, and these males have no major congenital defects. Triploidy with 69 chromosomes typically leads to fetal loss, but hydrops and hygroma are not features of this condition.