USMLE PRACTICE QUESTION #58
TOPIC: Respiratory pathophysiology
Today’s question will test your understanding of Respiratory pathophysiology.
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A 23-year-old male presents to the emergency department with symptoms of headache, nausea, and lightheadedness. He reports that the headache has persisted for five days, although he tends to feel better in the mornings. He also reports that he has been experiencing chest tightness and dyspnea. Vital signs are all within normal limits. To tolerate the cold winter months, he has been working on fixing a classic car in his garage for the past couple of weeks. Which of the following is most consistent with a diagnosis of carbon monoxide poisoning?
A. Pa02 normal, Sa02 ↑, total 02 ↓, [Hb] normal
B. Pa02 normal, Sa02 ↓, total 02 ↑, [Hb] ↑
C. Pa02 normal, Sa02 normal, total 02 ↓, [Hb] ↓
D. Pa02 normal, Sa02 normal, total 02 normal, [Hb] ↑
E. Pa02 normal, Sa02 ↓, total 02 ↓, [Hb] normal
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. Here’s how to think through this question:
Step 1. Read the last line to determine what the question is asking: Which option is consistent with a diagnosis of carbon monoxide poisoning?
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order: 1st. Recognize the signs and symptoms of CO poisoning 2nd. Know which metrics will reflect this pathology.
Step 3. Read the vignette to gather information.
Step 4. Consider the question and ask yourself the most likely answer.
Step 5. Look at the answer choices and select the option most closely resembling your thoughts from “Step 4” above.
GENERAL ANALYSIS
In the scenario described, the patient is likely experiencing carbon monoxide (CO) poisoning. CO binds to hemoglobin with a much higher affinity than oxygen, forming carboxyhemoglobin, which reduces the blood's oxygen-carrying capacity and impairs oxygen delivery to tissues.
ANSWER CHOICES:
ANSWER CHOICE A: PaO2 normal, SaO2 ↑, total O2 ↓, [Hb] normal
Explanation: This scenario is unlikely because SaO2 (arterial oxygen saturation) would not be increased in CO poisoning; it would be falsely elevated on pulse oximetry but not actually increased.
ANSWER CHOICE B: PaO2 normal, SaO2 ↓, total O2 ↑, [Hb] ↑
Explanation: This combination is inconsistent with CO poisoning. In CO poisoning, the total O2 would not be increased.
ANSWER CHOICE C: PaO2 normal, SaO2 normal, total O2 ↓, [Hb] ↓
Explanation: This does not fit CO poisoning as SaO2 would be falsely normal or decreased due to carboxyhemoglobin.
ANSWER CHOICE D: PaO2 normal, SaO2 normal, total O2 normal, [Hb] ↑
Explanation: This does not describe CO poisoning since total O2 would be affected by the presence of carboxyhemoglobin.
ANSWER CHOICE E: PaO2 normal, SaO2 ↓, total O2 ↓, [Hb] normal
Explanation: This is consistent with CO poisoning. PaO2 remains normal because it measures dissolved oxygen in plasma. SaO2 is decreased due to carboxyhemoglobin formation reducing available hemoglobin for oxygen binding. Total O2 content is decreased because of reduced effective hemoglobin for oxygen transport.
FINAL VERDICT…
In carbon monoxide poisoning:
• PaO2 remains normal because it reflects the blood's partial pressure of dissolved oxygen.
• SaO2 appears decreased due to carboxyhemoglobin, which displaces oxygen from hemoglobin.
• Total O2 content is decreased because the effective hemoglobin available for oxygen transport is reduced.
• [Hb] remains normal unless other underlying conditions affect hemoglobin levels.
This answer aligns with the pathophysiological effects of carbon monoxide binding to hemoglobin and impairing oxygen delivery despite normal PaO2 levels.
CORRECT ANSWER: E) PaO2 normal, SaO2 ↓, total O2 ↓, [Hb] normal
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Thanks for checking out Q58!
See you next time 👋