Which of the following is the most specific marker for Neural Tube Defects?

Acetylcholinesterase level in the amniotic fluid has got better diagnostic value than alpha feto protein in the diagnosis of neural tube defects. Acetylcholinesterase is more neural tissue specific and is 99-100% accurate, and remain so after 20weeks gestation when amniotic fluid alfa fetoprotein has risen and become more difficult to interpret. Ref: William's, 23rd Edition, Page 291; Tan Donald's Practical Obstetric Problem By Renu Mishra, 6th Edition, Page 44; Maternal Fetal Medicine : Principles and Practice, 5th Edition, Page 256; Textbook of Obstetrics By DC Dutta, 6th Edition, Page 59, 107.