FCOG-1163

A 26-year-old gravida 2 para 1 presents at 30 weeks gestation with a complaint of severe itching. She has excoriations from scratching in various areas. She says that she had the same problem during her last pregnancy, and her medical records reveal a diagnosis of intrahepatic cholestasis of pregnancy. Elevation of which one of the following is most specific and sensitive marker of this disorder?