Pyloric stenosis is a 3 to 6 week old with nonbilious projectile vomiting, an olive-shaped epigastric mass, and a hypochloremic, hypokalemic metabolic alkalosis. Midgut volvulus from malrotation is a newborn, usually in the first month of life, with sudden bilious vomiting, and it's a surgical emergency. Intussusception is a child between 6 months and 3 years with episodes of colicky pain, drawn-up legs, a sausage-shaped mass, and currant-jelly stool as a late finding. Age and the color of the vomit separate them before you read anything else.
All three show up in the same kind of stem: a baby who is vomiting and a parent who is worried. The question writer knows you'll see "vomiting infant" and jump to whichever one you studied last. The answer is sitting in five details you can pull out in under a minute: age, what the vomit looks like, what the stool looks like, what the belly feels like, and the named imaging sign.
Why do boards put these three in the same stem?
Because the chief complaint is identical. "An infant is brought to the emergency department for vomiting" could open any of the three, and each one has a different next step. One goes to the operating room tonight, one goes to the operating room after a day of IV fluids, and one gets an enema in radiology.
They also borrow each other's buzzwords. Pyloric stenosis and intussusception both produce a "target" or "doughnut" on transverse ultrasound. The "string sign" belongs to pyloric stenosis on an upper GI series and to Crohn disease on a small bowel follow-through. Intussusception vomit starts nonbilious and turns bilious later, so it can look like volvulus if the stem catches it late.
The board task is to sort the stem using features that don't overlap. Age comes first, emesis character second, and the rest confirms.
Intussusception vs volvulus vs pyloric stenosis side by side
This is the table to know cold. Every row is a separate axis the question writer can test.
| Feature | Pyloric stenosis | Midgut volvulus (malrotation) | Intussusception |
|---|---|---|---|
| Classic age | 3 to 6 weeks (range about 2 to 8 weeks) | First month of life, often the first week | 6 months to 3 years, most under 2 |
| Vomit | Nonbilious, projectile, after every feed | Bilious (green), sudden onset | Nonbilious early, bilious later |
| Behavior | Hungry right after vomiting | Irritable, then lethargic as bowel dies | Episodes of screaming with legs drawn up, calm or lethargic between |
| Stool | Small and infrequent | Bloody stool means ischemia (late) | Currant-jelly stool, blood and mucus (late) |
| Abdominal exam | Olive-shaped mass in epigastrium, visible peristaltic waves | Can be benign early, then distended and tender | Sausage-shaped mass in the right upper quadrant, empty right lower quadrant |
| Key labs | Hypochloremic, hypokalemic metabolic alkalosis | Metabolic acidosis and rising lactate once ischemic | Often normal early |
| First imaging | Abdominal ultrasound | Upper GI contrast series | Abdominal ultrasound |
| Classic sign | Thick, long pylorus; string sign and shoulder sign on upper GI | Corkscrew duodenum, duodenojejunal junction right of midline; whirlpool sign on ultrasound | Target or doughnut sign (transverse), pseudokidney sign (longitudinal) |
| Treatment | Fix fluids and electrolytes, then pyloromyotomy | Emergency Ladd procedure | Air or contrast enema reduction; surgery if it fails or perforates |
If you can fill this out from memory, most of these questions take less than a minute.
What makes pyloric stenosis different?
Hypertrophic pyloric stenosis is a thickening of the pyloric muscle that narrows the gastric outlet. The obstruction sits proximal to the ampulla of Vater, so bile never reaches the stomach and the vomit is never green. That single anatomic fact is the most tested point in the whole topic.
The baby is hungry, and the timing is specific
The classic stem is a firstborn boy around 4 weeks old. He feeds well, then vomits forcefully across the room, then wants to eat again right away. Boys are affected about four times as often as girls. The vomiting usually starts after the second week of life, so a baby vomiting on day 1 doesn't fit.
Look for macrolide exposure. Erythromycin or azithromycin given in the first 2 weeks of life, often as pertussis prophylaxis, raises the risk, and the AAP Red Book flags it for exactly that reason. If a stem mentions a sick older sibling with whooping cough and a newborn who got erythromycin, it's telling you the diagnosis.
The exam and the labs carry the diagnosis
On exam you may feel a firm, mobile, olive-shaped mass in the epigastrium or right upper quadrant, best felt right after the baby vomits. You may also see peristaltic waves moving left to right across the upper abdomen. The baby is often dehydrated and losing weight.
The labs are a classic acid-base question. The baby vomits hydrochloric acid, which leaves bicarbonate behind and drops chloride. Volume loss turns on aldosterone, which holds sodium at the cost of potassium and hydrogen ions. The result is a hypochloremic, hypokalemic metabolic alkalosis. Late in the course the kidney excretes hydrogen ions to save sodium, which produces paradoxical aciduria. The full workup for this pattern is in the acid-base disorder approach for boards.
Imaging and treatment
Ultrasound is the test of choice. It shows a thick pyloric muscle (3 mm or more) and a long pyloric channel (about 15 mm or more). On a transverse view the thick muscle around the collapsed channel looks like a target or doughnut, which is why you can't diagnose intussusception from "target" alone. An upper GI series is the older answer, showing the string sign (a thin trickle of contrast through the channel) and the shoulder sign.
Pyloromyotomy is the definitive treatment, but it isn't the next step. Correct the dehydration and the alkalosis first with IV fluids that contain dextrose, and add potassium once the baby is making urine. Uncorrected alkalosis raises the risk of apnea after anesthesia. If an answer choice sends a hypochloremic baby straight to the operating room, it's a distractor.
What makes midgut volvulus different?
Malrotation is a failure of normal midgut rotation. During development the midgut herniates into the umbilical cord around week 6, rotates 270 degrees counterclockwise around the superior mesenteric artery, and returns to the abdomen around week 10. When that goes wrong, the bowel sits on a narrow mesenteric base that can twist, and fibrous Ladd bands can cross and compress the duodenum.
Bilious vomiting in a newborn is an emergency
The twist usually happens at the duodenum, distal to the ampulla, so the vomit is green. Most cases present in the first month of life. The stem reads something like "a 5-day-old who was feeding well suddenly vomits bright green fluid."
Treat that phrase as malrotation with volvulus until proven otherwise. The twist cuts off the superior mesenteric artery, and the entire midgut, from the duodenum to the mid transverse colon, can die within hours.
The exam can look reassuringly normal
Early on, the belly can be soft and flat because the obstruction is high. That's the trap. A benign exam doesn't rule out volvulus. As ischemia sets in, the abdomen distends and becomes tender, the baby passes bloody stool, and labs shift toward metabolic acidosis with a rising lactate. By the time the stem describes peritonitis and shock, the next step is the operating room with no imaging.
Imaging and treatment
In a stable baby, the upper GI contrast series is the test of choice. Normally the duodenojejunal junction (the ligament of Treitz) sits to the left of the spine at the level of the pylorus. In malrotation it sits to the right of midline or too low. With volvulus, the duodenum and proximal jejunum take a corkscrew course, or the contrast stops in a tapered bird's beak.
Ultrasound may show the whirlpool sign, which is the superior mesenteric vein and mesentery wrapped around the artery. It may also show the vein sitting to the left of the artery, which is reversed. A plain film can be normal or show a dilated stomach and duodenum with little gas beyond.
The treatment is an emergency Ladd procedure. The surgeon untwists the bowel counterclockwise, divides the Ladd bands, widens the mesenteric base, removes the appendix, and leaves the small bowel on the right and the colon on the left. Start IV fluids and NG decompression on the way, but don't let them delay surgery.
The pediatric volvulus is not the coffee bean
"Volvulus" in an adult stem usually means sigmoid volvulus, an older patient with chronic constipation and a coffee bean sign on X-ray. Cecal volvulus is the other adult version. Neither belongs in a newborn stem. Both are covered in the volvulus section of high-yield abdominal imaging for boards.
What makes intussusception different?
Intussusception is one segment of bowel telescoping into the next, most often the terminal ileum into the cecum (ileocolic). The trapped mesentery gets compressed, which causes venous congestion, swelling, bleeding from the mucosa, and eventually ischemia. It's the most common cause of bowel obstruction in children between 6 months and 3 years.
The lead point changes with age
In the typical toddler, there's no structural lead point. Enlarged Peyer patches after a viral illness, often adenovirus, act as the lead. Older children and adults usually have a pathologic lead point. Meckel diverticulum is the classic one, followed by polyps and lymphoma.
Two associations get tested repeatedly. IgA vasculitis (Henoch-Schönlein purpura) causes bowel wall hemorrhage that can lead the telescope, and those cases are often ileoileal. Rotavirus vaccine carries a small increased risk, concentrated in the first week after the first dose, and a history of intussusception is a contraindication to the vaccine.
The pain comes in waves
The stem describes a toddler who suddenly screams, pulls the knees to the chest, and then settles. The cycle repeats every 15 to 20 minutes. Between episodes the child may look well or strangely lethargic. Lethargy alone can be the presenting sign, and boards use that to make the stem look like a neurologic problem.
Vomiting starts nonbilious and can turn bilious as the obstruction progresses. Currant-jelly stool is a mix of blood and mucus from the congested mucosa. It's a late finding, and most children don't have the full triad of pain, vomiting, and currant-jelly stool at once. Don't wait for the stool to make the diagnosis.
On exam, look for a sausage-shaped mass in the right upper quadrant and an empty-feeling right lower quadrant, because the cecum has been pulled up and over.
Imaging and treatment
Ultrasound is the test of choice. The transverse view shows the target or doughnut sign, rings of bowel wall inside bowel wall. The longitudinal view shows the pseudokidney sign. A plain film is often normal or shows a paucity of gas on the right.
Air or contrast enema reduction under imaging guidance is both diagnostic and therapeutic in a stable child. The contraindications are peritonitis, shock, and free air, and any of those sends the child to surgery. Perforation during an enema shows up as free air, and those patterns are in pneumoperitoneum on imaging. Intussusception recurs in roughly 10% of children after a successful enema, so a returning symptom pattern days later is the same diagnosis again.
How do you work through the vomiting infant stem?
The stem clues framework from the Skool curriculum puts age and acuity at the top of every question, and this differential is one of the cleanest examples. Here's the order to read it in.
- Age first. A newborn in the first weeks points toward volvulus or atresia. A baby 3 to 6 weeks old points toward pyloric stenosis. A child between 6 months and 3 years points toward intussusception.
- Color of the vomit. Green means the obstruction is past the ampulla. Nonbilious means it sits at or above the pylorus, or it's early intussusception.
- Acuity. Weeks of worsening vomiting with weight loss is pyloric stenosis. A sudden change in a baby who was feeding well is volvulus. Episodic pain over hours is intussusception.
- Stool. Currant jelly is intussusception. Frank blood in a newborn with bilious vomiting is a dying midgut.
- Labs and imaging. The objective findings are king. A hypochloremic alkalosis locks in pyloric stenosis. A named sign usually locks in the rest.
- Read the question part slowly. "Most likely diagnosis," "best initial test," and "next step in management" have different answers for the same baby. In pyloric stenosis the diagnosis is surgical, but the next step is IV fluids.
Common board pitfalls and how to avoid them
Pitfall 1: Calling every target sign intussusception. Pyloric stenosis also makes a target on transverse ultrasound. Check the age and the location. A 4-week-old with a target at the gastric outlet has pyloric stenosis.
Pitfall 2: Reassuring yourself with a soft belly. A newborn with bilious vomiting and a benign exam still needs an urgent upper GI series. Early volvulus often looks fine on exam.
Pitfall 3: Sending pyloric stenosis to surgery first. Fluids and electrolyte correction come before pyloromyotomy. The operation isn't an emergency, and the alkalosis is.
Pitfall 4: Waiting for currant-jelly stool. It's a late sign of mucosal ischemia. Episodic colicky pain with drawn-up legs in a toddler is enough to order the ultrasound.
Pitfall 5: Mixing up the double bubble. Duodenal atresia gives a double bubble with no gas beyond it, usually on the first day of life, often in a baby with Down syndrome and a history of polyhydramnios. A double bubble with some distal gas in a sick newborn should make you think of volvulus instead.
Pitfall 6: Confusing the two string signs. The string sign in a vomiting infant is a narrow pyloric channel. The string sign in a teenager with diarrhea and weight loss is a stenotic terminal ileum, covered in Crohn disease vs ulcerative colitis buzzwords.
Pitfall 7: Mistaking reflux for obstruction. A thriving infant who spits up small amounts effortlessly after feeds has physiologic reflux. Projectile vomiting with weight loss and dehydration does not fit reflux.
If you keep landing on the wrong one of two answers in questions like these, how to choose between two answers on COMLEX walks through the point system I teach for exactly that situation.
Frequently asked questions about intussusception vs volvulus vs pyloric stenosis
What is the fastest way to tell these three apart on a board question?
Start with age and the color of the vomit. A baby 3 to 6 weeks old with nonbilious projectile vomiting is pyloric stenosis. A newborn in the first month with sudden bilious vomiting is malrotation with midgut volvulus until proven otherwise. A child between 6 months and 3 years with episodes of colicky pain and drawn-up legs is intussusception. Then confirm with the exam finding (olive versus sausage-shaped mass), the stool, and the named imaging sign.
Why is pyloric stenosis vomiting nonbilious?
The obstruction is at the pylorus, which sits proximal to the ampulla of Vater where bile enters the duodenum. Bile never reaches the stomach, so the vomit is milk or gastric contents without green color. Midgut volvulus usually obstructs the duodenum distal to the ampulla, so bile backs up into the stomach and the vomit is green. That anatomy is why bilious vomiting in a newborn always raises concern for a surgical emergency.
What electrolyte abnormality does pyloric stenosis cause?
Pyloric stenosis causes a hypochloremic, hypokalemic metabolic alkalosis. Repeated vomiting removes hydrochloric acid from the stomach, which leaves bicarbonate behind. The volume loss activates aldosterone, which saves sodium while the kidney loses potassium and hydrogen ions. Late in the course, the kidney excretes acid despite the alkalosis, called paradoxical aciduria. The alkalosis has to be corrected with IV fluids before pyloromyotomy because it raises the risk of apnea after anesthesia.
What is the test of choice for bilious vomiting in a newborn?
In a stable newborn, the upper GI contrast series is the test of choice for malrotation with midgut volvulus. It shows the duodenojejunal junction to the right of midline or too low, and a corkscrew duodenum when volvulus is present. Ultrasound can support the diagnosis with the whirlpool sign. If the baby has peritonitis, shock, or bloody stool, the answer is emergency surgery (the Ladd procedure) without waiting for imaging.
When is an enema the wrong answer for intussusception?
An air or contrast enema is the first-line treatment for a stable child with intussusception, and it reduces the telescoped bowel while confirming the diagnosis. It's the wrong answer when the child has peritonitis, shock, or free air on imaging, which suggest perforation or dead bowel. Those children need surgery. Surgery is also the answer after a failed enema reduction or when an older child likely has a pathologic lead point such as a Meckel diverticulum.
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