Final answer:
The most common cause of uterine atony in the first 24 hours postpartum is retained placental fragments.
Step-by-step explanation:
The most common cause of uterine atony in the first 24 hours postpartum is a. Retained placental fragments. When fragments of the placenta remain in the uterus, they can cause postpartum hemorrhage. It is important for the obstetrician to examine the expelled placenta and fetal membranes to ensure they are intact and to avoid complications.