Question

Which of the following is the most specific marker for neural tube defects?

• A. Acetylcholinesterase
• B. Pseudocholinesterase
• C. Alpha fetoprotein (AFP)
• D. Human chorionic gonadotrophin (HCG)

Hint:

AFP screens, but which neural enzyme in amniotic fluid actually confirms an open NTD?

Answer 1

The Correct Option is A

Step 1: Separate screening from confirmation. In an open neural tube defect, fetal proteins leak into the amniotic fluid and then into maternal serum. Maternal serum AFP rises, which is why it is used to screen, but a raised AFP has many other causes.

Step 2: List why AFP is sensitive but not specific. Maternal serum AFP also rises with open abdominal wall defects, multiple pregnancy, wrong gestational dating, and fetal demise. So AFP flags the problem but cannot confirm it.

Step 3: Bring in the confirmatory marker. When AFP is high, amniotic fluid is tested for acetylcholinesterase (AChE). AChE is a neural enzyme that escapes only when neural tissue is exposed, as in an open neural tube defect. Its presence in amniotic fluid is far more specific for an open NTD than AFP.

Step 4: Reject the rest. Pseudocholinesterase is a liver-derived enzyme unrelated to NTD. HCG is a placental marker used in aneuploidy screening, not for NTD.

Answer: Acetylcholinesterase (Option A).