USMLE® Step 1 style questions USMLE
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A 28-year-old woman, gravida 2, para 1, comes to the clinic for her routine prenatal ultrasonography at 20 weeks’ after gestation. She has been trying to have a baby for the past three years. She reports no recent changes in her health, and that she does have unilateral renal agenesis. The fetal ultrasound shows bilateral renal agenesis. Which of the following clinical findings is most likely to be seen upon the birth of the child?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
So in Potter sequence there’s basically very little amniotic fluid and this makes the uterus a pretty hostile place for the fetus to grow.
Typically, around the 20th week of gestation, the kidneys, ureters, and urethra develop and start producing and excreting urine, and this urine becomes the major source of amniotic fluid.
Various conditions like renal agenesis, which is when one or both kidneys are missing, or atresia of the ureter or urethra, which is where those parts of the urinary tract are obstructed, can lead to an inability to produce or excrete urine and therefore leads to oligohydramnios.
Other common causes can include amniotic rupture, which is the leakage of amniotic fluid, or uteroplacental insufficiency, which is where there’s low blood flow from the placenta meaning the fetal organs—which includes the fetal kidneys—see less blood flow, which in turn leads to decreased urine production.
When there’s very little amniotic fluid, a couple of things happen as a result. First, amniotic fluid is crucial for the development of the fetal lungs, by both helping the airways physically stretch out as well as contributing amino acids like proline, which helps with the formation of connective tissue and collagen in the lung.
With less amniotic fluid, though, there’s pulmonary hypoplasia. Hypo meaning under, and -plasia means formation, so the lungs basically remain underdeveloped.
Not only that though, with less amniotic fluid, there’s less space in the amniotic sac, and so the fetus is literally compressed into a smaller space, which causes developmental abnormalities like a flattened face, wrinkly skin, widely separated eyes with epicanthal folds, low-set ears, as well as limb abnormalities like clubbed feet.