USMLE Step 2 CK OB/GYN Questions: Placenta Previa, Eclampsia and Magnesium Toxicity

by

Third-trimester emergencies are high-yield on USMLE Step 2 CK. You need to know what to do first, what not to do, and how to recognize medication toxicity. Try these three questions, then open the explanations.

Question 1: Painless bright red bleeding at 34 weeks

A 32-year-old woman, gravida 3, para 2, at 34 weeks’ gestation comes to the labor and delivery unit because of painless, bright red vaginal bleeding that started 1 hour ago. She has had no prenatal care. Her blood pressure is 118/72 mm Hg and pulse is 92/min. The uterus is soft and nontender. Fetal heart rate tracing is reassuring. Which of the following is the most appropriate next step?

A. Digital cervical examination
B. Transabdominal ultrasonography
C. Immediate cesarean delivery
D. Oxytocin infusion
E. Amniotomy

Correct answer: B. Transabdominal ultrasonography

Painless third-trimester bleeding with a soft, nontender uterus suggests placenta previa. The first step is ultrasonography to locate the placenta (transabdominal first; transvaginal is safe and more accurate if needed). A digital cervical exam is contraindicated until previa is excluded, because it can cause massive hemorrhage. With a stable mother and reassuring fetal status, management is expectant, with planned cesarean delivery.

Why the other options are wrong:

  • Digital exam can trigger severe bleeding in placenta previa.
  • Immediate cesarean is for unstable maternal or fetal status; this patient is stable.
  • Oxytocin to induce labor is inappropriate; vaginal delivery is contraindicated with previa.
  • Amniotomy requires a cervical exam and is contraindicated.

Question 2: Severe hypertension, headache and a seizure at 36 weeks

A 28-year-old primigravid woman at 36 weeks’ gestation has a severe headache. Her blood pressure is 168/112 mm Hg on two readings 15 minutes apart, and urinalysis shows 3+ protein. While being evaluated, she has a generalized tonic-clonic seizure. Which of the following is the most appropriate drug to prevent further seizures?

A. Lorazepam
B. Phenytoin
C. Labetalol
D. Magnesium sulfate
E. Levetiracetam

Correct answer: D. Magnesium sulfate

This is eclampsia: preeclampsia with new-onset seizures. Magnesium sulfate is the drug of choice to prevent and treat eclamptic seizures and is more effective than phenytoin or benzodiazepines in this setting. Severe-range blood pressure is treated with labetalol, hydralazine or oral nifedipine. The definitive treatment is delivery once the mother is stabilized.

Why the other options are wrong:

  • Lorazepam may be used for a seizure that does not stop despite magnesium, but it is not first-line prevention.
  • Phenytoin is less effective than magnesium for eclampsia.
  • Labetalol controls blood pressure but does not prevent seizures.
  • Levetiracetam is not the treatment of choice in eclampsia.

Question 3: Absent reflexes and slow breathing during treatment

A 30-year-old woman at 32 weeks’ gestation with preeclampsia with severe features is receiving an intravenous magnesium sulfate infusion. Six hours later she is drowsy. Her respiratory rate is 8/min, and patellar reflexes are absent. Urine output has decreased over the past 4 hours. Which of the following is the most appropriate next step?

A. Stop the magnesium infusion and give intravenous calcium gluconate
B. Increase the magnesium infusion rate
C. Give intravenous naloxone
D. Give intravenous labetalol
E. Give intravenous furosemide only

Correct answer: A. Stop the magnesium infusion and give intravenous calcium gluconate

These are signs of magnesium toxicity. Toxicity follows a predictable order as levels rise: loss of deep tendon reflexes first, then respiratory depression, and finally cardiac arrest. Magnesium is cleared by the kidneys, so decreasing urine output increases the risk. Management is to stop the infusion and give IV calcium gluconate as the antidote, with respiratory support as needed. This is why reflexes, breathing and urine output are monitored during magnesium therapy.

Why the other options are wrong:

  • Increasing magnesium would worsen the toxicity.
  • Naloxone reverses opioids, not magnesium.
  • Labetalol lowers blood pressure and does nothing for magnesium toxicity.
  • Furosemide alone does not reverse life-threatening toxicity quickly.

Key takeaways

  • Painless third-trimester bleeding = placenta previa: ultrasound first, never a digital exam first.
  • Eclampsia: magnesium sulfate prevents seizures; delivery is the definitive treatment.
  • Magnesium toxicity: lost reflexes, then respiratory depression; stop the infusion and give calcium gluconate.

More USMLE practice questions

Repeated USMLE Questions Step 2 CK Review Vol 1 book cover

Want 100+ more clinical questions like these?

Get Repeated USMLE Questions Step 2 CK Review Vol 1 by Dr. Ray Makar

Buy on Amazon →

As an Amazon Associate, I earn from qualifying purchases.

Written and reviewed by Dr. Ray Makar, MD, dermatologist with 20 years of clinical practice. Last reviewed: October 2026. For exam preparation only, not medical advice.

FREE USMLE PDFs

Get 80 free practice questions

Four free sampler PDFs by Dr. Ray Makar: Step 1 (Pathology & Pharmacology) and Step 2 CK (Medicine, Surgery & Pediatrics), in both Review style and USMLE style, with answers and explanations. Also useful for NEET-PG, INI-CET and FMGE.

Download the free PDFs See the full books →

Leave a Comment