Which finding in a 12-month-old infant should a nurse report to the provider?

The correct choice is that the nurse should report to the provider when the birth weight has doubled. While it is expected for a baby to have weight gain as they grow, by 12 months, typically the birth weight will have tripled, not just doubled. Doubling the birth weight at this age might indicate potential issues such as inadequate nutrition or a growth disorder, and it warrants further investigation by the provider.

On the other hand, the closure of the anterior fontanel is expected around this age; it usually closes between 9 to 18 months. The eruption of six teeth is also a normal finding for a 12-month-old, as babies often have several teeth by this age. Lastly, an increase in birth length by 50% is a typical growth milestone for infants and doesn't raise red flags for health concerns. Thus, only the finding of the birth weight doubling requires immediate concern and further evaluation.