What is the most important assessment finding of a newborn that requires follow-up after a vaginal delivery?

A tuft of hair at the base of the spine in a newborn is a significant assessment finding that warrants follow-up. This is often associated with underlying conditions, such as spina bifida, a common neural tube defect where the spinal column does not close completely. The presence of a tuft of hair can indicate that there may be structural abnormalities beneath the skin, such as defects in the spinal column or spinal cord.

In contrast, the other findings are generally considered more benign. A flat bluish discolored area on the buttocks may represent a Mongolian spot, which is a type of birthmark that is typically harmless. Localized soft tissue edema of the scalp, known as caput succedaneum, can occur during vaginal delivery due to pressure from the birth canal and usually resolves on its own. A small amount of whitish substance in the axilla could be a remnant of vernix caseosa, which is normal and does not require follow-up.

Therefore, the tuft of hair stands out as the most critical finding that necessitates further evaluation due to its potential association with serious underlying issues.