A 29-year-old woman delivers vaginally at term after an uncomplicated pregnancy. Thirty minutes later, she develops heavy vaginal bleeding; estimated blood loss is 1100 mL, blood pressure 90/60 mm Hg, uterus is boggy on palpation. According to SOGC postpartum haemorrhage guidance, what is the most appropriate first-line pharmacologic uterotonic after basic resuscitation has begun?