USMLE Step 1 · Embryology & Anatomy

Posterior neural tube defects vs Anterior neuropore defect and severe open NTDs

How to tell Posterior neural tube defects and Anterior neuropore defect and severe open NTDs apart when a board question makes them look alike.

Fastest separator

Posterior neuropore defects localize to spine; anterior neuropore failure causes anencephaly.

Side by side

Posterior neural tube defectsAnterior neuropore defect and severe open NTDs

Key clue

Spina bifida occulta, meningocele, and myelomeningocele reflect posterior neuropore closure defects with vertebral arch defects; myelomeningocele includes neural tissue in the sac.

Anencephaly reflects anterior neuropore failure with absent cranial vault/forebrain; severe open NTDs raise maternal AFP and acetylcholinesterase.

Mechanism

Posterior neuropore closure failure leaves spinal elements open to varying depth: occult bony defect, meningocele with meninges, or myelomeningocele with meninges plus neural tissue.

Anterior neuropore closure failure prevents normal cranial vault and forebrain formation, producing anencephaly rather than a spinal-level sac.

Labs and tests

Posterior NTDs may show elevated maternal AFP when open; occult lesions can have a sacral hair tuft or dimple with normal AFP.

Anencephaly/open NTDs cause markedly elevated maternal AFP and amniotic acetylcholinesterase.

The trap

Occulta is hidden vertebral defect; meningocele is meninges only; myelomeningocele includes neural tissue; anencephaly is anterior neuropore failure.

The repair move

Before naming the answer, force the stem through the separator: Occulta is hidden vertebral defect; meningocele is meninges only; myelomeningocele includes neural tissue; anencephaly is anterior neuropore failure.

Try a free question

Core

A pregnancy is complicated by markedly elevated maternal serum AFP and acetylcholinesterase. Fetal ultrasound shows absent cranial vault with frog-like facies; brain stem and cerebellum are visible but the cerebral hemispheres are absent. Which neural tube defect is most likely?

Show answer and explanation

Answer: A. Anencephaly. Anencephaly is failure of the ANTERIOR (rostral) neuropore closure during the 4th week of gestation. Results in absent cranial vault and forebrain. Markedly elevated maternal AFP and acetylcholinesterase. Incompatible with prolonged extrauterine life. Strongly associated with maternal folate deficiency; folate supplementation reduces risk.

Why the other choices are tempting

B. Spina bifida occulta is the mildest posterior NTD with intact skin over a vertebral arch defect, often a sacral hair tuft or dimple; normal AFP.
C. Meningocele is a posterior NTD with meninges (no neural tissue) in a sac through a vertebral defect.
D. Myelomeningocele is a posterior NTD with meninges PLUS neural tissue in a sac.
E. Encephalocele is a midline cranial defect with brain herniation but is less severe than anencephaly.

Flashcards

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What clue supports Spina bifida occulta and meningocele?

Choose Spina bifida occulta and meningocele when the stem gives this pattern: Spina bifida occulta is vertebral arch defect with skin tuft/dimple; meningocele herniates meninges without spinal cord tissue.

What clue supports Myelomeningocele and anencephaly?

Choose Myelomeningocele and anencephaly when the stem gives this contrasting pattern: Myelomeningocele herniates meninges and spinal cord with neurologic deficits; anencephaly is failure of anterior neuropore closure with absent forebrain/skull.

Quick answers

What is the fastest way to tell Posterior neural tube defects from Anterior neuropore defect and severe open NTDs?