What condition is indicated by single palmar creases in a newborn?

The presence of single palmar creases, often referred to as a "simian line," is a notable feature associated with Trisomy 21, also known as Down syndrome. This characteristic is part of the phenotypic presentation in individuals with this genetic condition.

In the context of Trisomy 21, the single palmar crease reflects an abnormality in the way the fetal hand develops in utero due to the extra chromosome. Most typically, individuals will have two distinct creases on their palms, but a single crease can suggest the presence of the syndrome, especially when considered alongside other clinical features, such as hypotonia, a flat facial profile, and characteristic slanted eyes.

While single palmar creases might arise as a normal genetic variation in some individuals, they are predominantly recognized in the context of Down syndrome when combined with other findings. Therefore, while it is essential to consider all possibilities, the most clinically significant association remains with Trisomy 21. Conditions like infections and metabolic disorders do not typically present with this hallmark sign.