USMLE® Step 1 style questions USMLE
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A 4-week-old boy comes to the emergency department because of continuous postprandial non-bilious vomiting. He has continued to lose weight since birth and has become increasingly lethargic. The mother denies any complications with pregnancy or birth and denies any recent sick contacts. His temperature is 37.0°C (98.6°F), pulse is 130/min, respirations are 35/min, and blood pressure is 70/45 mm Hg. The child is lethargic with dry mucous membranes and a sunken anterior fontanelle. A palpable mass is felt in the epigastric region, and a succussion splash is heard on auscultation of the abdomen. Which of the following metabolic imbalances is most likely found in his laboratory results?
Content Reviewers:Rishi Desai, MD, MPH
Contributors: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.
The vomit is also non-bilious, meaning it doesn’t contain bile, which makes sense, since bile secretion happens after the pyloric sphincter in the duodenum.