Final answer:
The most likely cause of a sudden gush of bleeding in a postpartum client is uterine atony, which occurs when the uterus fails to contract after birth, leading to excessive bleeding. Uterine contractions help to seal off blood vessels following the delivery of the placenta, and breastfeeding can assist in this process.
Step-by-step explanation:
The nurse observes a postpartum client experiencing a sudden gush of bleeding. The most likely cause of the bleeding is a) Uterine atony, which is a condition where the uterus fails to contract effectively after birth. When the uterus doesn't contract, this can lead to excessive bleeding. This condition is often related to the fact that after the delivery of the placenta, continued uterine contractions are needed to help seal off blood vessels where the placenta was attached. Breastfeeding can aid the process of involution, where the uterus returns to its pre-pregnancy size and location, and helps to reduce postpartum bleeding. Though other options such as b) Vaginal laceration, c) Cervical tear, and d) Lochia expulsion are potential causes, uterine atony is the most probable immediately following childbirth if there's a significant gush of bleeding.