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
The concentration of lecithin relative to sphingomyelin begins to rise in the following week: 
Select and explain. 
A. 20 wks, B. 32 wks, C. 36 wks, D. 28 wks
B: 32 wks
Answer: B: 32 wks 
Explanation: 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.