NBME Obstetrics and Gynecology 3 — Questions and Answers
Question 1: A 26-year-old G1P0 at 16 weeks undergoes amniocentesis. Results show markedly elevated amniotic fluid AFP and a positive acetylcholinesterase. What is the most likely diagnosis?
- Trisomy 21 (Down syndrome)
- Open neural tube defect (Correct answer)
- Trisomy 18
- Turner syndrome (45,X)
Correct answer: Open neural tube defect
Elevated amniotic AFP combined with positive acetylcholinesterase is diagnostic of an open neural tube defect, confirming fetal neural tissue is exposed to amniotic fluid.
Question 2: A 24-year-old at 28 weeks with gestational diabetes has a growth ultrasound showing estimated fetal weight at the 97th percentile with a disproportionately large abdominal circumference. This finding is most directly caused by which fetal mechanism?
- Fetal hyperinsulinemia causing increased fat deposition (Correct answer)
- Fetal hyperglycemia causing osmotic fluid shifts
- Placental insufficiency causing growth acceleration
- Fetal glucagon excess stimulating hepatic glycogen
Correct answer: Fetal hyperinsulinemia causing increased fat deposition
Maternal hyperglycemia leads to fetal hyperinsulinemia, which acts as a growth factor causing macrosomia through increased fat and glycogen deposition.
Question 3: A 34-year-old G2P1 at 11 weeks has first-trimester screening showing nuchal translucency of 4.2 mm, elevated free β-hCG, and low PAPP-A. This pattern is most consistent with which chromosomal abnormality?
- Trisomy 21 (Down syndrome) (Correct answer)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Turner syndrome (45,X)
Correct answer: Trisomy 21 (Down syndrome)
Increased NT, elevated free β-hCG, and decreased PAPP-A is the classic first-trimester combined screen pattern for trisomy 21.
Question 4: A 29-year-old Rh-negative woman at 28 weeks has a negative antibody screen and has not previously received Rh immunoglobulin. What is the appropriate management?
- Administer Rh immunoglobulin now and again within 72 hours of delivery (Correct answer)
- Obtain Kleihauer-Betke test to guide dosing
- Amniocentesis for fetal blood typing before deciding
- No intervention until delivery unless a sensitizing event occurs
Correct answer: Administer Rh immunoglobulin now and again within 72 hours of delivery
Unsensitized Rh-negative women should receive 300 mcg Rh immunoglobulin at 28 weeks and again postpartum to prevent alloimmunization.
Question 5: A 30-year-old woman at 36 weeks is GBS-positive and has a reported history of anaphylaxis to penicillin. GBS susceptibilities are available. What is the correct intrapartum antibiotic prophylaxis regimen?
- Clindamycin IV if the isolate is susceptible (Correct answer)
- Ampicillin IV (safe with documented anaphylaxis)
- Cefazolin IV (first-line for all penicillin allergies)
- Azithromycin IV
Correct answer: Clindamycin IV if the isolate is susceptible
For penicillin allergy with high anaphylaxis risk, clindamycin is used if the GBS isolate is susceptible; if resistant or unknown, vancomycin is given.
Question 6: A 31-year-old G3P2 at 38 weeks undergoes a nonstress test. Over 20 minutes, the baseline FHR is 138 bpm with two accelerations of 15 bpm lasting 15 seconds each. How should this tracing be interpreted?
- Reactive — reassuring fetal status (Correct answer)
- Nonreactive — requires further evaluation
- Suspicious for fetal metabolic acidosis
- Indicates immediate delivery
Correct answer: Reactive — reassuring fetal status
A reactive NST requires at least 2 accelerations of ≥15 bpm lasting ≥15 seconds within a 20-minute window, which this tracing meets.
Question 7: A 22-year-old primigravida at 32 weeks has BP 158/106 mmHg, blurred vision, and 3+ proteinuria. Magnesium sulfate is administered. What is the primary therapeutic goal of magnesium in this clinical scenario?
- Reduce maternal blood pressure to safe levels
- Prevent eclamptic seizures (Correct answer)
- Accelerate fetal lung maturity
- Inhibit uterine contractions
Correct answer: Prevent eclamptic seizures
Magnesium sulfate is used as seizure prophylaxis (eclampsia prevention) in severe-range preeclampsia; it is not an effective antihypertensive agent.
NBME Obstetrics and Gynecology 3
A 26-year-old G1P0 at 16 weeks undergoes amniocentesis.
Results show markedly elevated amniotic fluid AFP and a positive acetylcholinesterase.
What is the most likely diagnosis?