USMLE Step 1 Question 317

Question:

An infant born at 27 weeks develops respiratory distress syndrome. In the lungs, alveoli are maintained in their open, expanded state with the help of pulmonary surfactant which helps to reduce surface tension. Which of the following describes the composition of pulmonary surfactant after the 36th week of pregnancy in normal development?

Correct Answer: D.  Surfactant containing phosphatidylglycerol

Pulmonary surfactant after the 36th week of pregnancy contains phosphatidylglycerol and this is the best sign of maturity of the fetal lungs. In addition, a lecithin-to-sphingomyelin ratio of greater than 2:1 is a marker that the fetus will not develop respiratory distress syndrome and is seen later in the third trimester. Mature pulmonary surfactant contains 75% lecithin and 10% phosphatidylglycerol, not sphingomyelin.

Immature pulmonary surfactant contains a higher proportion of sphingomyelin and the species of lecithin that is less surface active, alpha-palmitate, beta-myristate.