USMLE PRACTICE QUESTION #83
TOPIC: Pulmonary pathology & physiology
Today’s question will test your understanding of the pulmonary physiology & pathology.
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A 2-year-old female was brought to the emergency department due to persistent coughing and respiratory distress. The onset of these symptoms occurred while the child was playing with her older brother. Upon visual inspection, the patient appears to be anxious and in distress. The observed respiratory rate is recorded at 28 breaths per minute and her oxygen saturation, measured on room air, is 85%. Upon physical examination, intercostal retractions and nasal flaring is evident. Auscultation reveals absent breath sounds on the left side, accompanied by a high-pitched inspiratory wheeze. Despite the administration of 100% oxygen, the patient's symptoms fail to improve. Given these findings, which of the following best explains the cause of her hypoxemia?
A) Limited diffusion
B) Increased physiologic dead space
C) Decreased affinity of hemoglobin for oxygen
D) Ventilation-perfusion mismatch
E) Increased pulmonary vascular resistance
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: “Given these findings, which of the following best explains the cause of her hypoxemia?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Diagnose the condition, and 2nd: Identify the underlying cause of the condition.
Step 3: Read the vignette carefully and ask yourself: “What’s the underlying reason for this patient’s condition?”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
The clinical presentation of the 2-year-old female with sudden respiratory distress, decreased oxygen saturation, and absent breath sounds on one side suggests a foreign body aspiration.
ANSWER CHOICES:
CHOICE A: Limited diffusion
Explanation: This refers to impaired gas exchange at the alveolar-capillary level, often seen in interstitial lung disease. However, this is not consistent with the acute onset and unilateral findings.
CHOICE B: Increased physiologic dead space
Explanation: This occurs when areas of the lung are ventilated but not perfused, such as in pulmonary embolism. The issue here is not lack of perfusion but obstruction of ventilation.
CHOICE C: Decreased affinity of hemoglobin for oxygen
Explanation: This would result in less oxygen being carried by hemoglobin. This does not explain the acute respiratory symptoms and unilateral findings.
CHOICE D: Ventilation-perfusion mismatch
Explanation: This occurs when parts of the lung receive oxygen without blood flow or blood flow without oxygen. In this case, the obstruction prevents ventilation, leading to hypoxemia. The foreign body causes a blockage in airflow, creating a mismatch where perfusion is present but ventilation is not, explaining the hypoxemia.
CHOICE E: Increased pulmonary vascular resistance
Explanation: This is typically related to conditions like pulmonary hypertension. It does not account for the acute unilateral absence of breath sounds and hypoxemia.
FINAL VERDICT…
The likely cause of hypoxemia in this scenario is a ventilation-perfusion mismatch due to a foreign body obstructing airflow on one side. This results in lung areas being perfused but not ventilated, leading to reduced oxygenation despite supplemental oxygen.
CORRECT ANSWER: D) Ventilation-perfusion mismatch
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That’s it for question 83!
See ya tomorrow 👋