USMLE Step 2 CK Pediatrics Questions: Pyloric Stenosis, Intussusception and Midgut Volvulus

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Vomiting in an infant is a common USMLE Step 2 CK scenario. The age, whether the vomit is bilious, and the stool findings point to the diagnosis, and the exam asks for the next best step. Try these three questions.

Question 1: A 4-week-old with forceful vomiting after every feed

A 4-week-old firstborn boy has had forceful, nonbilious vomiting after every feeding for 5 days. He seems hungry right after vomiting. He has dry mucous membranes. A small, firm, mobile mass is felt in the epigastrium. Ultrasonography shows a thickened, elongated pylorus. Serum chloride is 88 mEq/L, potassium 3.1 mEq/L and bicarbonate 34 mEq/L. Which of the following is the most appropriate next step?

A. Immediate pyloromyotomy
B. Upper GI contrast study
C. Intravenous fluid resuscitation and correction of electrolytes
D. Start a proton pump inhibitor and thicken feeds
E. Air enema

Correct answer: C. Intravenous fluid resuscitation and correction of electrolytes

This is hypertrophic pyloric stenosis: projectile nonbilious vomiting at 2–6 weeks of age, an “olive” mass, and hypochloremic, hypokalemic metabolic alkalosis from loss of gastric acid. The diagnosis is confirmed by ultrasound. Pyloric stenosis is not a surgical emergency: first correct dehydration and electrolytes with IV fluids, then perform pyloromyotomy. Operating with uncorrected alkalosis increases anesthesia risk, including postoperative apnea.

Why the other options are wrong:

  • Immediate pyloromyotomy comes only after fluids and electrolytes are corrected.
  • Upper GI series is unnecessary because ultrasound has already confirmed the diagnosis.
  • PPI and thickened feeds are used for reflux, not an obstruction.
  • Air enema is used to treat intussusception.

Question 2: Episodes of crying with legs drawn up, then a bloody stool

An 18-month-old boy has had episodes of severe, crampy abdominal pain for 12 hours. During each episode he screams and draws his knees up to his chest; between episodes he is drowsy. He has vomited twice and just passed a stool that looks like red currant jelly. A sausage-shaped mass is felt in the right upper abdomen. He is hemodynamically stable with no signs of peritonitis. Ultrasonography shows a “target sign.” Which of the following is the most appropriate treatment?

A. Air or contrast enema
B. Exploratory laparotomy
C. Intravenous antibiotics and observation
D. Upper GI contrast study
E. Colonoscopy

Correct answer: A. Air or contrast enema

This is intussusception, most common between 6 months and 3 years, usually at the ileocecal junction. Classic features are intermittent colicky pain with legs drawn up, lethargy, a sausage-shaped mass and “currant jelly” stool (a late sign). Ultrasound shows a target sign. In a stable child without perforation or peritonitis, an air or contrast enema is both diagnostic and therapeutic. Surgery is reserved for failed reduction, perforation or peritonitis.

Why the other options are wrong:

  • Laparotomy is for unstable patients, peritonitis, perforation or failed enema reduction.
  • Antibiotics and observation do not reduce the intussusception and risk bowel necrosis.
  • Upper GI series is used to diagnose malrotation, not intussusception.
  • Colonoscopy has no role in this emergency.

Question 3: Sudden green vomiting in a 10-day-old

A 10-day-old girl, previously feeding well, suddenly develops repeated bright green vomiting. She is irritable, and her abdomen is mildly distended. Vital signs are normal. Abdominal x-ray shows a few dilated loops of bowel without free air. Which of the following is the most appropriate next step?

A. Abdominal ultrasonography for pyloric thickness
B. Observation and repeat x-ray in 24 hours
C. Barium enema
D. Switch to a soy-based formula
E. Emergency upper GI contrast study

Correct answer: E. Emergency upper GI contrast study

Bilious vomiting in a newborn is a surgical emergency until proven otherwise. The key concern is malrotation with midgut volvulus, which can cut off blood supply to the entire midgut within hours. The test of choice in a stable infant is an emergency upper GI contrast study, which shows the duodenojejunal junction in an abnormal position, or a “corkscrew” appearance with volvulus. Treatment is emergency surgery (Ladd procedure). If the infant is unstable or has peritonitis, go directly to surgery.

Why the other options are wrong:

  • Pyloric ultrasound is for nonbilious vomiting; pyloric stenosis vomit is never bilious.
  • Observation risks necrosis of the entire midgut.
  • Barium enema is less reliable for diagnosing malrotation than an upper GI series.
  • Changing formula is appropriate for milk protein intolerance, not obstruction.

Key takeaways

  • Pyloric stenosis: nonbilious projectile vomiting, hypochloremic hypokalemic metabolic alkalosis; fix fluids first, then pyloromyotomy.
  • Intussusception: currant jelly stool and target sign; air or contrast enema if stable.
  • Bilious vomiting in a newborn = malrotation with volvulus until proven otherwise; emergency upper GI series.

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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.

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