Welcome to USMLE practice question #145! Today’s topic is respiratory physiology.
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A 28-year-old man travels to a high-altitude region (12,000 feet above sea level) for a hiking expedition. Within 24 hours of arrival, he develops a headache, nausea, dizziness, and dyspnea on exertion. On physical examination, his oxygen saturation is 86% on room air. Arterial blood gas analysis reveals hypoxemia and respiratory alkalosis. Which of the following is the primary mechanism responsible for the development of his symptoms?
A) Hypoxic pulmonary vasoconstriction
B) Increased capillary permeability in the lungs
C) Decreased alveolar partial pressure of oxygen
D) Impaired hemoglobin oxygen unloading in tissues
E) Hyperventilation-induced hypocapnia
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
It’s important to adopt the correct MENTAL MODEL when answering USMLE questions; it saves time and increases accuracy. The mental model outlined below is a foundational component of our 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 is the primary mechanism responsible for the development of his symptoms?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Diagnose the problem, 2nd: Identify the underlying cause.
Step 3: Read the vignette carefully and ask yourself: “My patient’s current condition is caused by ______________________.”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
This 28-year-old man develops acute mountain sickness after rapidly ascending to a high-altitude region. His symptoms of headache, nausea, dizziness, dyspnea on exertion, and hypoxemia (oxygen saturation of 86%) with respiratory alkalosis are classic for this condition.
ANSWER CHOICES:
CHOICE A: Hypoxic pulmonary vasoconstriction
Explanation: Hypoxic pulmonary vasoconstriction occurs in response to low alveolar oxygen levels and increases pulmonary vascular resistance. It contributes to complications like high-altitude pulmonary edema (HAPE) but is not the primary cause of AMS symptoms.
CHOICE B: Increased capillary permeability in the lungs
Explanation: Increased capillary permeability is associated with high-altitude pulmonary edema (HAPE), which presents with severe dyspnea at rest, cough, and pink frothy sputum.
CHOICE C: Decreased alveolar partial pressure of oxygen
Explanation: Barometric pressure decreases with altitude → Reduced PIO₂ → Decreased PAO₂ → Hypoxemia.
Hypoxemia triggers hyperventilation via peripheral chemoreceptors → Respiratory alkalosis.
CHOICE D: Impaired hemoglobin oxygen unloading in tissues
Explanation: Respiratory alkalosis shifts the oxygen-hemoglobin dissociation curve to the left, reducing oxygen unloading in tissues. However, this is a secondary effect and does not explain the primary hypoxemia caused by reduced PAO₂.
CHOICE E: Hyperventilation-induced hypocapnia
Explanation: Hyperventilation-induced hypocapnia causes respiratory alkalosis and contributes to some AMS symptoms like dizziness or paresthesias. However, it is a compensatory response to hypoxemia rather than the primary cause.
FINAL VERDICT…
CORRECT ANSWER: C) Decreased alveolar partial pressure of oxygen
The primary mechanism responsible for acute mountain sickness is decreased alveolar partial pressure of oxygen (PAO₂) at high altitude due to reduced barometric pressure.
KEY CONCEPTS:
High-Altitude Physiology:
Barometric pressure decreases with altitude → Reduced PIO₂ → Decreased PAO₂ → Hypoxemia.
Hypoxemia triggers hyperventilation via peripheral chemoreceptors → Respiratory alkalosis.
Symptoms of Acute Mountain Sickness (AMS):
Headache, nausea, dizziness.
Dyspnea on exertion.
Fatigue and insomnia.
Diagnostic Features:
Hypoxemia with low PaO₂.
Respiratory alkalosis (low PaCO₂ and elevated pH).
Management of AMS:
Gradual ascent for acclimatization.
Supplemental oxygen or descent for severe symptoms.
Acetazolamide: Enhances renal bicarbonate excretion to correct alkalosis and improve ventilation.