Pyloric stenosis


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Pyloric stenosis

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Pyloric stenosis


0 / 9 complete

USMLE® Step 1 questions

0 / 1 complete

High Yield Notes

8 pages


Pyloric stenosis

of complete


USMLE® Step 1 style questions USMLE

of complete

A 3-week-old infant boy is brought to the emergency room for evaluation of non-bilious vomiting that is precipitated by feeding. He was born at 38-weeks gestational age to a 23-year-old, gravida 1 para 1, woman via a vaginal delivery. During pregnancy, the patient’s mother was diagnosed with Chlamydial cervicitis and prescribed azithromycin, which successfully treated the infection. Physical exam is notable for sunken anterior and posterior fontanelles. Abdominal exam is notable for a firm mass palpated in the right upper quadrant. Laboratory testing is ordered. Which of the following findings will most likely be present? 

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Hypertrophic pyloric stenosis p. 368

Macrolides p. 190

hypertrophic pyloric stenosis and p. 368

Metabolic alkalosis p. 610, 616

in hypertrophic pyloric stenosis p. 368


hypertrophic pyloric stenosis in p. 368

Pyloric stenosis p. 368


pyloric stenosis p. 368

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Content Reviewers

Rishi Desai, MD, MPH


Tanner Marshall, MS

With hypertrophic pyloric stenosis, hypertrophy refers to an increase in size, pyloric refers to the pylorus which is the tissue between the stomach and the duodenum, and stenosis means narrowing, so hypertrophic pyloric stenosis, or HPS, is a congenital condition where a baby’s pylorus grows in size such that it narrows the tiny opening between the stomach and the duodenum.

The pylorus itself has two parts to it, the pyloric antrum, which connects to the body of the stomach, and the pyloric canal, which connects to the duodenum.

At the end of the pyloric canal you’ve got the pyloric sphincter, which is a ring of smooth muscle that contracts and acts like a valve, letting food pass down into the duodenum, but not go back up into the stomach.

In HPS, babies are born with a normal pylorus, but within a few weeks after birth, the smooth muscle of the pyloric antrum begins to undergo hypertrophy and hyperplasia, meaning an increase in the size of each cell as well as an increase in the overall number of cells, respectively.

This causes the pyloric antrum to nearly double in size.

This thick and muscular antrum obstructs the pathway of food, which makes it harder for food to leave the stomach and enter the small intestine.

Clinically the enlarged pylorus can be felt as an “olive” in the right upper quadrant or epigastric region of the abdomen, which is just above the umbilicus.

Also, there’s normally contraction and relaxation of the smooth muscle lining the stomach, a process called peristalsis.

Obstruction from HPS can cause the stomach smooth muscle to have to work much harder to push food through, and sometimes there can even be hypertrophy of those muscles, which can result in peristalsis that can be felt or seen.

If food can’t pass through the pylorus, it quickly starts to build up to the point where it has nowhere to go, which can lead to vomiting.

This usually happens around 2-6 weeks, and can get more intense over time, until it ultimately starts causing projectile vomiting, called that because the vomit literally launches out of a child’s mouth.


Pyloric stenosis is the narrowing of the opening from the stomach to the duodenum, often caused due to hypertrophy of the muscle surrounding this opening, which spasms when the stomach empties. Pyloric stenosis causes severe projectile nonbilious vomiting after meals, abdominal pain, poor weight gain, and dehydration. It usually presents in the first few months of life, and the thickened pylorus can be felt classically as an olive-shaped mass in the middle upper part or right upper quadrant of the infant's abdomen. Pyloric stenosis can be treated with pyloromyotomy, a surgical procedure that enlarges the pylorus.


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  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
  4. "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
  5. "Pyloric stenosis of infancy and primary hyperacidity - the missing link" Acta Paediatrica (2014)
  6. "Diagnosis and Therapy of Primary Hypertrophic Pyloric Stenosis in Adults: Case Report and Review of Literature" Journal of Gastrointestinal Surgery (2006)

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