A 32-year-old pregnant woman (G2P1) presents for routine prenatal care at 17 weeks of gestation, confirmed by her last menstrual period and an earlier ultrasound. She has no significant medical or genetic history, and her first pregnancy was uncomplicated, resulting in the delivery of a healthy baby. Her family history is also unremarkable for genetic disorders or birth defects. During this visit, she undergoes a second-trimester maternal serum screening (quadruple screen) as part of routine prenatal care. What is the primary marker associated with neural tube defects in maternal serum screening?
