Final answer:
The baby with a karyotype showing three copies of chromosome 18 most likely has Trisomy 18, which is associated with congenital cardiac malformations along with other physical abnormalities.
Step-by-step explanation:
The condition described in the question, with the baby having three copies of chromosome 18, is known as Trisomy 18 or Edwards Syndrome. Among the symptoms listed, congenital cardiac malformations are most commonly associated with Trisomy 18. This chromosomal disorder leads to a high rate of mortality in the early stages of life, and those who survive often have severe intellectual and developmental disabilities. Babies with Trisomy 18 may present various physical abnormalities, including those described in the question, such as micrognathia, malformed ears, ocular hypertelorism, syndactyly, and clenched hands with overlapping fingers, along with congenital heart defects.