Final answer:
Prior to administering methylergonovine for uterine subinvolution, the nurse should assess the patient's blood pressure to ensure the safety of using the medication, as it can cause hypertension.
Step-by-step explanation:
Before administering methylergonovine, a medication used to control bleeding by contracting the uterus postpartum, the nurse should assess the patient's Blood pressure. This is because methylergonovine can cause hypertension as a side effect, and assessing the blood pressure is crucial to ensure it is safe to give the medication, especially in someone with a potential or known history of hypertension. It is less critical to check the respiratory rate, temperature, or urinary output prior to giving this specific medication, although those assessments may be part of routine postpartum care.