USMLE PRACTICE QUESTION #95
TOPIC: Principles of Respiratory Physiology
Today’s question will test your understanding of a high-yield concept of respiratory physiology.
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A 49-year-old male with a history of hypertension presents to the emergency department complaining of dyspnea and chest pain. He has a 20-pack-year smoking history and reports unilateral swelling of the right calf, which is confirmed on physical examination. Vital signs reveal a heart rate of 110/minute and a respiratory rate of 20/minute. A chest x-ray shows a wedge-shaped opacity in the peripheral aspect of the left lung. Based on his symptoms and exam findings, which of the following statements is most accurate?
A) V/Q ratio will be decreased
B) A-a gradient will be normal
C) Administration of 100% 02 will improve Pa02
D) Atelectasis is a highly specific finding in his condition
E) ST-segment elevation is likely to be observed on ECG
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: “Based on his symptoms and exam findings, which of the following statements is most accurate?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Diagnose the condition; 2nd: Recognize the correlated physiological abnormality.
Step 3: Read the vignette carefully and ask yourself: “Based on my assessment of the situation, what physiological change do I expect to see?”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
This vignette presents a 49-year-old male with risk factors for pulmonary embolism (PE), including hypertension, smoking history, and unilateral calf swelling (suggestive of deep vein thrombosis). His symptoms of dyspnea and chest pain, along with the chest X-ray showing a wedge-shaped opacity (Hampton's hump), are classic signs of PE.
ANSWER CHOICES:
CHOICE A: V/Q ratio will be decreased
Explanation: The ventilation/perfusion (V/Q) ratio represents the balance between air reaching the alveoli (ventilation) and blood reaching the alveoli (perfusion). In a pulmonary embolism, blood flow to a portion of the lung is blocked, leading to a high V/Q ratio in the affected area. In PE, the V/Q ratio is increased in the embolized region because ventilation is normal, but perfusion is reduced or absent.
CHOICE B: A-a gradient will be normal
Explanation: The alveolar-arterial (A-a) gradient measures the difference between oxygen concentration in the alveoli and arterial blood. Normally, this gradient increases in conditions that impair gas exchange, such as pulmonary embolism. In PE, the A-a gradient is typically increased due to impaired oxygenation from mismatched ventilation and perfusion. Although a normal A-a gradient is possible in some cases of PE, it is uncommon and should not be expected.
CHOICE C: Administration of 100% O2 will improve PaO2
Explanation: Administering 100% oxygen can improve arterial oxygen levels (PaO2) in many conditions by increasing the amount of dissolved oxygen in the blood. Although there is a V/Q mismatch in pulmonary embolism, areas of the lung that are still ventilated and perfused can benefit from supplemental oxygen. Administering 100% O2 can increase PaO2 by improving oxygenation in unaffected lung regions.
CHOICE D: Atelectasis is a highly specific finding in his condition
Explanation: Atelectasis refers to the collapse of lung tissue, which can occur due to various causes, such as airway obstruction or external compression. While atelectasis can occur with PE due to reduced surfactant production or hypoventilation in affected lung regions, it is not a specific finding for PE. Other conditions like pneumonia or pleural effusion can also cause atelectasis.
CHOICE E: ST-segment elevation is likely to be observed on ECG
Explanation: ST-segment elevation on an ECG typically indicates myocardial ischemia or infarction. While ECG changes like sinus tachycardia or right heart strain may be seen in PE, ST-segment elevation is not commonly associated with PE unless there is concurrent myocardial ischemia. This answer would be more appropriate for acute coronary syndromes.
FINAL VERDICT…
Pulmonary embolism causes a mismatch between ventilation and perfusion due to obstruction of blood flow to parts of the lung. This results in areas where ventilation continues but perfusion is impaired (high V/Q ratio), leading to reduced oxygenation. However, areas of the lung that remain ventilated and perfused can still participate in gas exchange. Administering 100% oxygen increases the partial pressure of oxygen in these functional areas, thereby improving overall arterial oxygen levels (PaO2). While supplemental oxygen cannot fully correct the underlying V/Q mismatch caused by PE, it can significantly enhance oxygen delivery to tissues by increasing dissolved oxygen in the blood.
CORRECT ANSWER: C) Administration of 100% O2 will improve PaO2
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That’s it for question 95!
See ya tomorrow 👋