Ans. is 'a' & 'b' i.e., a fetoprotein & Acetylcholinesterase * In intra embryonic life, neural tube is open at both end and freely communicate with amniotic cavity. Failure of closure of neural tube results in persistent of this communication. This allows excretion of following fetal substances into amniotic cavity -1) Alpha - fetoprotein2) Acetylcholinesterase* These serve as biochemical markers for NTDs for prenatal diagnosis.* In patients with elevated amniotic fluid AFP to differentiate between NTD and abdominal wall defect, acetylcholinesterase and pseudocholinesterase are measured.MarkerNeural tube defectAbdominal wall defectAcetylcholinesteraseHighLowPseudocholinesteraseLowHighRatio of Ach/ pseudocholinesterase>0.14<0.14