This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'Biochemical analytes of combined test of first-trimester screening is \nSelect and explain. \nA. AFP and b-hcg, B. b-hcg and PAPP-A, C. AFP and PAPP-A, D. Inhibin and PAPP-A', 'role': 'user'}, {'content': "Answer: B \nExplanation: Two analytes used for first-trimester aneuploidy screening are human chorionic gonadotropin and pregnancy-associated plasma protein A (PAPP-A). In cases of fetal Down syndrome , the first-trimester serum free b-hcg level is higher, approximately 2.0 MoM, and the PAPP-A level is lower, approximately 0.5 MoM. With trisomy 18 and trisomy 13, levels of both analytes are lower.Reference: William's Obstetrics; 24th edition; Chapter 14; Prenatal diagnosis", 'role': 'assistant'}]