What assessment finding may confirm a diagnosis of septic abortion?

A boggy uterus with rebound pain is indicative of septic abortion due to the presence of retained products of conception or a uterine infection, which can lead to uterine atony. In this context, "boggy" refers to the uterus being soft and less firm, which is characteristic of an infection or inflammatory process. Rebound tenderness indicates peritoneal irritation, often associated with underlying infection or inflammation, making it a significant finding for diagnosing septic abortion.

The other choices would not support a diagnosis of septic abortion effectively. Normal uterine tone suggests a lack of complications, while uterine rigidity may imply more of a traumatic event rather than an infectious process. The detection of a fetal heartbeat would typically indicate a viable pregnancy rather than septic abortion. Thus, the findings of a boggy uterus with rebound pain provide crucial information consistent with the diagnosis of septic abortion.