A 32-year-old primigravida at 28 weeks gestation presents with sudden onset vaginal bleeding, severe abdominal pain, and uterine tenderness. Fetal heart rate is 110 bpm with variable decelerations. Ultrasound confirms placental abruption with 40% placental separation. The patient is haemodynamically stable with normal coagulation profile. What is the drug of choice for fetal neuroprotection in this preterm scenario?