Question

Which is the best marker for an open neural tube defect?

• A. Acetylcholinesterase
• B. Pseudocholinesterase
• C. Alpha-fetoprotein (AFP)
• D. hCG

Hint:

AFP screens, but a neural-specific enzyme confirms an open defect.

Answer 1

The Correct Option is A

Step 1: Understand what an open neural tube defect (NTD) is. When the neural tube fails to close (open spina bifida or anencephaly), exposed fetal neural tissue and cerebrospinal fluid leak substances into the amniotic fluid.
Step 2: Distinguish the two key markers. Maternal serum and amniotic AFP are raised in open NTDs, but AFP also rises in many other conditions (abdominal wall defects, multiple gestation, fetal demise, wrong dates), so it is a sensitive screening marker, not the most specific.
Step 3: Identify the confirmatory marker. Acetylcholinesterase (AChE) is a neural-specific enzyme. Its presence in amniotic fluid indicates leakage from open neural tissue and is far more specific for an open NTD than AFP.
Step 4: Apply the logic of the question. When asked for the BEST (most specific/confirmatory) marker for an open NTD, amniotic fluid acetylcholinesterase is chosen, used to confirm a positive AFP screen.
Step 5: Exclude the rest. Pseudocholinesterase is a liver enzyme used in suxamethonium apnoea, and hCG is a marker of trophoblastic tissue and Down syndrome screening, neither being specific for NTDs.
Hence the best marker is acetylcholinesterase.