This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'The concentration of lecithin relative to sphingomyelin begins to rise in the following week: \nSelect and explain. \nA. 20 wks, B. 32 wks, C. 36 wks, D. 28 wks', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Before 34 weeks, lecithin and sphingomyelin are present in amnionic fluid in similar concentrations. At 32 to 34 weeks, the concentration of lecithin relative to sphingomyelin begins to rise. Dipalmitoylphosphatidylcholine--DPPC, that is, lecithin--along with phosphatidylinositol and especially phosphatidylglycerol is an impoant component of the surface-active layer that prevents alveolar collapse. Ref: Leveno K.J., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 29. Diseases and Injuries of the Fetus and Newborn. In K.J. Leveno, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.', 'role': 'assistant'}]