USMLE PRACTICE QUESTION #48
TOPIC: Twin pregnancies.
Today’s question will test your understanding of twin pregnancies.
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 her fetuses?
A. Genetic differences between the twins causing varied growth rates
B. Unequal sharing of maternal nutrients and hormones
C. Disparity in amniotic sac size resulting in compression of the umbilical cord
D. Intrauterine growth restriction in one twin leads to compensatory growth in the other twin
E. Unequal sharing of placental circulation due to abnormal vascular connections
F. Delayed implantation of one blastocyst resulting in unequal placental development
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
The mental model for this question comes from our detailed test-taking skills masterclass (It was made for students who want to elevate their USMLE test-taking skills; if that’s you, check it out). Here’s how to think through this question:
Step 1. Determine what the question is asking: Identify the primary underlying physiological mechanism responsible for the discrepancies between the fetuses.
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order (1st. Find the problem, 2nd. Find the underlying cause).
Step 3. Read the vignette to figure out the diagnosis.
Step 4. Ask yourself what is responsible for the diagnosis.
Step 5. Look at the answer choices and select the option most closely resembling your thoughts from “Step 4” above.
GENERAL ANALYSIS
This question involves a 28-year-old pregnant woman with a twin pregnancy at 26 weeks of gestation. The ultrasound findings indicate significant differences in amniotic fluid volumes between the two twins, with Twin A showing signs of increased amniotic fluid, a larger bladder, and cardiac hypertrophy, while Twin B presents with decreased amniotic fluid, a smaller bladder, and growth restriction. The question asks for the primary underlying physiological mechanism responsible for these discrepancies. The clinical presentation suggests a condition affecting monochorionic twins, likely involving unequal blood flow or nutrient distribution.
ANSWER CHOICES:
ANSWER CHOICE A: Genetic differences between the twins causing varied growth rates
Genetic differences could theoretically lead to varied growth rates, but identical twins (monochorionic) share the same genetic material.
ANSWER CHOICE B: Unequal sharing of maternal nutrients and hormones
Unequal sharing of nutrients and hormones could cause growth differences, but this is typically not the primary mechanism in monochorionic twins.
ANSWER CHOICE C: Disparity in amniotic sac size resulting in compression of the umbilical cord
Compression of the umbilical cord could affect blood flow to a fetus, leading to growth restriction.
ANSWER CHOICE D: Intrauterine growth restriction in one twin leads to compensatory growth in the other twin
Intrauterine growth restriction (IUGR) in one twin could lead to compensatory growth in the other.
ANSWER CHOICE E: Unequal sharing of placental circulation due to abnormal vascular connections
In TTTS, abnormal vascular connections in the shared placenta lead to unequal blood flow, causing one twin (the donor) to have less blood and amniotic fluid, while the other (the recipient) has excess.
ANSWER CHOICE F: Delayed implantation of one blastocyst resulting in unequal placental development
Delayed implantation could theoretically cause differences in placental development.
FINAL VERDICT…
The condition described is Twin-to-Twin Transfusion Syndrome (TTTS), a complication of monochorionic twin pregnancies where there is an imbalance in blood flow due to abnormal vascular connections in the shared placenta. In TTTS, one twin (the donor) transfers blood to the other twin (the recipient). This results in the donor twin's reduced blood volume, leading to oligohydramnios (low amniotic fluid) and growth restriction. In contrast, the recipient twin receives excess blood, resulting in polyhydramnios (high amniotic fluid), a larger bladder, and cardiac hypertrophy due to increased workload. This condition is not related to genetic differences, nutrient sharing, or immune responses but rather to the unique vascular connections between the twins.
CORRECT ANSWER: E) Unequal sharing of placental circulation due to abnormal vascular connections
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