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USMLE® Step 1 Question: Neural Tube Defects

Osmosis Team
Published on Feb 3, 2021. Updated on Feb 16, 2021.

Each week, Osmosis shares a USMLE® Step 1-style practice question to test your knowledge of medical topics. Today's case involves a 2-hour old baby who was born with a defect and can't move his lower extremities. The mother is worried and wants to know if this could occur again if she has more children in the future. Can you help figure out what might have caused this birth defect?

A 2-hour-old boy is being evaluated in the nursery. He was born via spontaneous vaginal delivery to a 27-year-old woman who had inconsistent prenatal care. The mother’s medical history includes generalized seizures managed with carbamazepine. She took routine prenatal vitamins throughout the pregnancy. The mother has smoked 1 pack of cigarettes daily for the past 10 years, she does not use alcohol, and she used cocaine 1–2 times per month during the pregnancy.  Physical examination of the patient shows a newborn male in no acute distress. The patient moves the upper extremities freely but not the lower extremities. There is a red, fleshy, swelling present on the lower back. Stroking the skin around the anus does not produce a contraction of the anal sphincter. Which of the following interventions could have prevented this neonate’s abnormalities?

A. Cessation of cocaine use  

B. Smoking cessation 

C. Changing carbamazepine to valproate  

D. Increasing the dose of prenatal vitamins

 Scroll down to find the answer! 


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The correct answer to today's USMLE® Step 1 Question is...

D. Increasing the dose of prenatal vitamins 

This neonate is presenting with an abnormal lesion on the lower back, paraplegia, and loss of anal sphincter tone—a presentation consistent with meningomyelocele. 

Incorrect answer explanations

The incorrect answers to today's USMLE® Step 1 Question are...

A. Cessation of cocaine use 

Incorrect: Cocaine is a potent vasoconstrictor, and therefore, it can cause fetal and placental injury leading to placental abruption, spontaneous abortion, prematurity, and fetal death. Neural tube defects, as seen in this patient, are not associated with cocaine use.

B. Smoking cessation 

Incorrect: Tobacco use negatively affects both maternal and fetal health and continues over into the postnatal period. Tobacco use is associated with neonatal death, reduction in birth weight, placental abruption, placenta previa, and preterm birth. It is also associated with specific congenital malformations such as cleft lip with gastroschisis, cardiac defects, digital anomalies, bilateral renal agenesis, and anal atresia but not neural tube defects. 

C. Changing carbamazepine to valproate  

Incorrect: Both valproate and carbamazepine, as well as other folate antagonists (e.g., methotrexate), are associated with neural tube defects. Therefore, changing to valproate is unlikely to change the prognosis. 



Correct answer explanation

Open neural tube defects (NTDs) are congenital anomalies that develop when the neural tube fails to close normally during the third and fourth weeks of gestation. NTDs may be open (e.g., myelomeningocele, meningocele, encephalocele, and anencephaly) or closed (e.g. spina bifida occulta). 

Risk factors for developing closed or open neural tube defects include folic acid deficiency, gestational diabetes, maternal obesity, and certain medications (e.g., valproate, carbamazepine). Most isolated NTDs appear to be caused by folate deficiency, probably in combination with genetic or environmental risk factors. Folate deficiency may arise from inadequate oral intake, malabsorption, use of folic acid antagonists, or genetic causes of abnormal folate metabolism. Folic acid supplementation is recommended for all women during preconception and the first trimester periods; 0.4 mg daily of folic acid is recommended for all women, beginning before conception and continued through pregnancy. For women taking carbamazepine or valproate, or those with a prior infant with an NTD or with an NTD affecting either parent, a higher folic acid dose of 4 mg daily is recommended.

Major Takeaway

Neural tube defects are associated with folic acid deficiency, gestational diabetes, maternal obesity, and certain medications (e.g., valproate, carbamazepine). Women at risk should take an increased dose of folic acid before conception and throughout the pregnancy. 

References

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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.