A 30-year-old pregnant woman visits her obstetrician for prenatal care and eventual delivery. The patient volunteers that two of her three children had "yellow jaundice" at birth. Her youngest girl had been severely jaundiced and had been given two blood transfusions. Prenatal laboratory tests indicate that the mother is blood type O, Rh negative, whereas her husband is blood type A, Rh positive. The obstetrician samples amniotic fluid at 36 weeks of gestation to ascertain whether the fetus is mature enough for preterm delivery. Quantitative analysis of which of the following was most likely used as an indicator of fetal lung maturity?