A 35-year-old woman presents for her first prenatal visit at 12 weeks gestation. She has a family history of Down syndrome, with her sister having a child with the condition. Given her advanced maternal age and family history, the patient is offered first-trimester screening for Down syndrome. The patient undergoes a combined screening test, which includes a nuchal translucency ultrasound and maternal serum screening for pregnancy-associated plasma protein-A (PAPP-A), inhibin-A, and free beta-human chorionic gonadotropin (β-hCG). Which combination of maternal serum markers is most commonly associated with Down syndrome (Trisomy 21)?
