Step 1 topics → Gastrointestinal → Congenital Hypertrophic Pyloric Stenosis

Congenital Hypertrophic Pyloric Stenosis

USMLE Step 1 trap: Predicts metabolic acidosis in pyloric stenosis instead of hypochloremic metabolic alkalosis. Pyloric stenosis causes hypochloremic, hypokalemic metabolic alkalosis because vomiting loses HCl (not bile), and the kidneys retain bicarbonate while excreting H⁺ to preserve volume.

Congenital hypertrophic pyloric stenosis is tested on USMLE Step 1 from three angles, and students stumble on two of them. The lab pattern trips up students who confuse the direction of the acid-base disturbance: the vomit here is pure gastric HCl, not bicarbonate-rich fluid, so the result is hypochloremic, hypokalemic metabolic alkalosis — not acidosis. The management angle trips up students who default to 'obstruction = surgery now,' when the correct first step is IV fluid and electrolyte correction before pyloromyotomy. It classically presents in a 2–6 week old first-born male with projectile non-bilious vomiting and a palpable olive-shaped RUQ mass.

The lab pattern is where most students stumble. The vomit here is pure gastric contents — hydrochloric acid. The result is a hypochloremic, hypokalemic metabolic alkalosis. Students mix this up with other causes of vomiting, or confuse the direction of the acid-base disturbance. The kidney's compensatory response (paradoxical aciduria — excreting H⁺ to hold onto volume while bicarbonate rises) adds another layer that boards love to probe.

The management angle is equally high-yield and frequently tested. Many students default to 'obstruction = surgery now,' but that's the wrong instinct here. Operating on a severely alkalotic infant puts them at risk for apnea under anesthesia because alkalosis blunts the respiratory drive. USMLE Step 1 wants you to know that IV fluid resuscitation and electrolyte correction come first — pyloromyotomy (Ramstedt procedure) comes after stabilization.

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Common misconceptions

Common mistake
Wrong: Pyloric stenosis causes metabolic acidosis because the infant is vomiting and losing bicarbonate.
Right: Pyloric stenosis causes hypochloremic, hypokalemic metabolic alkalosis because vomiting loses HCl (not bile), and the kidneys retain bicarbonate while excreting H⁺ to preserve volume.
The key is knowing what's actually in the vomit. Pyloric stenosis causes loss of pure gastric acid (HCl) — not pancreatic or biliary secretions, which are alkaline and contain bicarbonate. Losing HCl means losing H⁺ and Cl⁻, which drives a metabolic alkalosis, not acidosis. The kidneys then compound the problem: to preserve volume, they hold onto Na⁺ and reabsorb bicarbonate while excreting H⁺ and K⁺ into urine (paradoxical aciduria) — pulling both potassium and pH further in the wrong direction.
Common mistake
Wrong: Pyloric stenosis requires immediate emergency surgery upon diagnosis.
Right: Pyloric stenosis is a medical emergency requiring IV fluid and electrolyte correction before surgery; operating on an alkalotic infant risks apnea under anesthesia.
Pyloric stenosis is a surgical disease, but it is not a surgical emergency in the acute sense — the timeline matters. The metabolic alkalosis from ongoing HCl loss suppresses respiratory drive, and if you put an alkalotic infant under general anesthesia before correcting their electrolytes, you risk apnea post-op. The correct sequence is always: IV fluid resuscitation (typically normal saline with dextrose), replace potassium, normalize the bicarbonate — then take the patient to the OR for pyloromyotomy.
Common mistake
Wrong: The vomiting in pyloric stenosis is bilious because the obstruction is near the duodenum.
Right: Vomiting in pyloric stenosis is non-bilious (projectile) because the obstruction is proximal to the ampulla of Vater, so bile cannot reflux into the stomach.
Bile enters the GI tract at the ampulla of Vater in the second part of the duodenum. The pylorus sits proximal to this point, so when it's obstructed, gastric contents can't move forward — but bile can't reflux backward into the stomach either. That's why pyloric stenosis causes forceful, non-bilious, non-green vomiting. Bilious (green) vomiting in a neonate should make you think distal to the ampulla: malrotation with volvulus, duodenal atresia (if below the ampulla), or jejunal/ileal obstruction.
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What the exam tests

  1. Recognize the classic presentation: a 2–6 week old male infant with progressively worsening projectile, non-bilious vomiting, visible peristaltic waves, and a palpable 'olive' mass in the right upper quadrant — and distinguish this from other causes of vomiting in infants.
  2. Identify and explain the characteristic lab pattern of hypochloremic, hypokalemic metabolic alkalosis — including why HCl loss (not bicarbonate loss) drives the alkalosis and how the kidneys worsen the picture through paradoxical aciduria.
  3. Know the correct management priority: IV fluid and electrolyte resuscitation must precede surgical correction (pyloromyotomy), because uncorrected alkalosis causes anesthesia-related apnea risk.

Can you avoid these mistakes?

A 4-week-old male presents with 1 week of worsening projectile vomiting after every feeding. His mother notes the vomit is never green. On exam, you palpate an olive-shaped mass. ABG shows pH 7.52, and labs show Na 138, K 3.0, Cl 88, HCO3 34. What is the diagnosis, and what is your FIRST intervention?
Why is the vomiting in pyloric stenosis non-bilious, while vomiting from malrotation with volvulus is typically bilious? What anatomical landmark explains the difference?
A student says: 'The infant is vomiting, so she must be losing bicarbonate and will have metabolic acidosis.' What is wrong with this reasoning, and what determines whether vomiting causes acidosis vs. alkalosis?
After correcting a pyloric stenosis infant's metabolic alkalosis with IV fluids, you notice urine pH is 5.2 despite the serum alkalosis. What is this phenomenon called, why does it occur in this condition, and when does it resolve?

Related topics

Gut Tube Regions and Blood Supply Midgut Rotation and Anomalies (Malrotation/Volvulus) Meckel Diverticulum Omphalocele vs Gastroschisis

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