Final answer:
The newborn girl's symptoms are characteristic of Down syndrome, most likely caused by nondisjunction that leads to trisomy 21. The closest option from the given choices is mosaicism, though it is not the classic cause for standard trisomy 21 presentation.
Step-by-step explanation:
The clinical findings described in the newborn girl are indicative of Down syndrome, also known as trisomy 21. This condition is caused by the presence of an extra copy of chromosome 21, which typically results from nondisjunction during cell division in the gametes of the parents. The features described, including mid-face hypoplasia, a flat nasal bridge, up-slanting palpebral fissures, a small mouth, and a single palmar crease, are characteristic of this syndrome. Additionally, the presence of a holosystolic murmur suggests a congenital heart defect, which is common in infants with Down syndrome.
The most likely genetic mechanism responsible for the infant's clinical findings is nondisjunction leading to an extra chromosome, which is not among the provided answer options. However, from the given choices, the mechanism that can result in an extra chromosome is mosaicism (Option B), although this doesn't fully align with the classic presentation of trisomy 21. Mosaicism in Down syndrome occurs when some cells in the body have three copies of chromosome 21 while others have the typical two copies.
The most likely cause of the infant's clinical findings is Down syndrome. Down syndrome is a genetic disorder that occurs when an individual has an extra copy of chromosome 21. The physical features described, such as mid-face hyperplasia, up-slanting palpebral fissures, and a single palmar crease, are common characteristics of Down syndrome. The blowing holosystolic murmur heard along the external border indicates the presence of a heart defect, which is also commonly seen in individuals with Down syndrome.