USMLE PRACTICE QUESTION #94
TOPIC: Cardiovascular Embryology
Today’s question will test your understanding of a super high-yield cardiac embryology concept.
I’m Paul Ciurysek, MD, founder of The USMLE Guys, and this is The USMLE Times. This daily newsletter aims to provide you with super high-yield USMLE concepts that are commonly tested on exam day. All content is FREE! If you’d like to work with me and my team, please see the options at the bottom of today’s newsletter. If you’d like to support my efforts, please share the newsletter with a friend.
A 32-year-old pregnant woman at 36 weeks’ gestation presents to her obstetrician for a prenatal visit. The patient is concerned about her baby's health and inquires about how the fetus receives oxygen and nutrients. The obstetrician explains the various adaptations present in the fetal circulation. Which of the following adaptations allows most of the oxygenated blood from the placenta to bypass the fetal liver and be preferentially directed to the fetal heart and brain?
A) Ductus arteriosus
B) Ductus venosus
C) Foramen ovale
D) Umbilical arteries
E) Umbilical vein
F) Inferior vena cava
G) Superior vena cava
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
The MENTAL MODEL used to answer this question comes from our detailed test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:
Step 1. Read the last line to get to the heart of the question: “Which of the following adaptations allows most of the oxygenated blood from the placenta to bypass the fetal liver and be preferentially directed to the fetal heart and brain?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 1st order. 1st: Identify the correct fetal circulation.
Step 3: Read the vignette carefully and ask yourself: “Which structure allows oxygenated blood to be directed to the heart and brain?”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
The patient's question concerns fetal circulation and how oxygenated blood is preferentially directed to the heart and brain while bypassing the liver. We must identify which fetal circulatory adaptation allows most of the oxygenated blood from the placenta to bypass the liver and flow directly into the inferior vena cava, which can then be directed to the fetal heart and brain.
ANSWER CHOICES:
CHOICE A: Ductus arteriosus
Explanation: The ductus arteriosus connects the pulmonary artery to the descending aorta, allowing blood to bypass the lungs. It does not play a role in bypassing the liver.
CHOICE B: Ductus venosus
Explanation: The ductus venosus is a shunt that allows oxygenated blood from the umbilical vein to bypass the liver and flow directly into the inferior vena cava. This ensures that highly oxygenated blood reaches vital organs like the heart and brain more efficiently.
CHOICE C: Foramen ovale
Explanation: The foramen ovale is a shunt between the right and left atria, allowing blood to bypass the lungs by moving from the right atrium to the left atrium. It does not affect liver circulation.
CHOICE D: Umbilical arteries
Explanation: The umbilical arteries carry deoxygenated blood from the fetus back to the placenta for gas exchange. They do not bypass any organs.
CHOICE E: Umbilical vein
Explanation: The umbilical vein carries oxygenated blood from the placenta to the fetus but does not bypass any organs on its own. The ductus venosus is responsible for directing this blood past the liver.
CHOICE F: Inferior vena cava
Explanation: The inferior vena cava carries mixed oxygenated and deoxygenated blood to the right atrium but does not specifically bypass any organs.
CHOICE G: Superior vena cava
Explanation: The superior vena cava carries deoxygenated blood from the upper body back to the right atrium. It does not play a role in bypassing organs or directing oxygenated blood.
FINAL VERDICT…
The ductus venosus is responsible for shunting most of the oxygenated blood from the placenta past the liver and towards vital organs like the fetal heart and brain, making it essential for efficient fetal circulation.
CORRECT ANSWER: B) Ductus venosus
1) 1-ON-1 STEP 1 COACHING: If you’re looking for a personalized approach and guidance designed for your specific needs, goals, and challenges, then 1-on-1 coaching might be what you’re looking for. Choose between daily sessions with Dr. Paul for four, six, or eight weeks. CLICK HERE TO LEARN MORE ABOUT 1-ON-1 COACHING.
2) 1-ON-1 STRATEGY SESSION: If you need some guidance or a plan to get to where you’re going, join me for a 1-on-1 strategy session. This will help you clarify a plan, identify and overcome potential challenges, and give you the best chance for a smooth ride from where you are now to where you’re going. These are done 1-on-1 with me (Dr. Paul), so space is limited. SET UP YOUR 1-ON-1 STRATEGY SESSION HERE
3) THE MASTER CLASSES: I've created two masterclasses based around two of the most commonly experienced challenges I've helped students overcome in the last 15+ years: Test-taking skills and How to get into Residency. These will give you the knowledge, skills, and tools to crush your USMLE exams and match into your dream Residency program. LEARN ABOUT THE MASTERCLASSES HERE
4) ASSESSMENT EXAMS: Save those valuable NBME assessments! Our assessment exam platform was designed to help you identify your strengths, weaknesses, and exam readiness without prematurely wasting your NBME exams. With our USMLEDx assessment platform, you can assess yourself early and often while saving those highly valuable NBMEs for the final stretch of your exam prep. CHECK OUT THE ASSESSMENT EXAM OPTIONS HERE
That’s it for question 94!
See ya tomorrow 👋