参考答案：B

解析：

Tetanic contractions are the most serious adverse effect associated with administering oxytocin. When tetanic contractions occur, the fetus is at high risk for hypoxia and the mother is at risk for uterine rupture. The client may be at risk for pulmonary edema if large amounts of oxytocin have been administered, and this drug can also increase blood pressure. However, pulmonary edema and increased blood pressure aren’t the most serious adverse effects. Early decelerations of fetal heart rate aren’t associated with oxytocin administration.