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
Biochemical analytes of combined test of first-trimester screening is 
Select and explain. 
A. AFP and b-hcg, B. b-hcg and PAPP-A, C. AFP and PAPP-A, D. Inhibin and PAPP-A
B: b-hcg and PAPP-A
Answer: B: b-hcg and PAPP-A 
Explanation: 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