USMLE STEP 1 QUESTION
TOPIC: Reproductive Pathology
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The following is a high-yield USMLE Step 1 question.
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Formulate your answer and find it in the answer choices
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QUESTION
A 28-year-old woman at 26 weeks’ gestation with a twin pregnancy is seeing her obstetrician for a routine prenatal visit. An ultrasound reveals significant differences in the amniotic fluid volume between the two gestational sacs. Twin A has a larger bladder, increased amniotic fluid, and signs of cardiac hypertrophy, while Twin B has a smaller bladder, decreased amniotic fluid, and growth restriction. What is the primary underlying physiological mechanism responsible for the discrepancies between the fetuses’?
A. Varied growth rates due to genetic differences
B. Unequal sharing of maternal nutrients and hormones
C. Disparity in amniotic sac size resulting in compression of the umbilical cord
D. Unequal sharing of placental circulation due to abnormal vascular connections
E. Delayed implantation of one blastocyst resulting in unequal placental development
F. Immune-mediated attack on the smaller twin's red blood cells
BIG-PICTURE:
The clinical picture described suggests a condition known as Twin-to-Twin Transfusion Syndrome (TTTS). This syndrome occurs in pregnancies with monochorionic twins (twins sharing the same placenta) due to unbalanced blood flow through vascular connections in the placenta between the twins.
In TTTS, the "donor" twin (Twin B in this scenario) pumps blood to the "recipient" twin (Twin A), leading to a surplus of amniotic fluid and potential cardiac overload in the recipient. In contrast, the donor twin has reduced amniotic fluid and might show signs of growth restriction.
CORRECT ANSWER: D. Unequal sharing of placental circulation due to abnormal vascular connections
Let's break down each answer choice and discuss the important concepts we should know for each ⬇️