Final answer:
The first thing a nurse should do after a patient's membranes rupture is to check the fetal heart rate (FHR), as it is an important indicator of the fetus's condition. Other secondary actions can be performed afterward to ensure the continued well-being of both mother and fetus.
Step-by-step explanation:
When a patient's membranes rupture, the immediate concern is potential distress or changes in the condition of the fetus. Thus, the first action that should be taken is c. check the fetal heart rate (FHR). The fetal heart rate is a critical indicator of the fetus's well-being, and changes in the FHR can be the first sign of distress. Monitoring the FHR ensures immediate action can be taken if the fetus shows any signs of compromise.
Other actions such as performing a Nitrazine test to confirm the rupture of membranes, performing a vaginal examination, and turning the patient onto her left side to optimize blood flow can be considered subsequently. However, checking FHR should be prioritized to safeguard the health of the fetus.