Asthma USMLE Step 1 Practice Question
A 26-year-old woman presents in winter with headache, dizziness, and nausea after using a faulty gas heater. Vital signs: BP 118/76, HR 102, RR 20, Temp 37.2°C, SpO2 98%. Physical exam reveals pink lips and no respiratory distress. Arterial blood gas shows normal PaO2 (95 mmHg) and carboxyhemoglobin level of 18%. She denies prior respiratory disease. Which mechanism best explains her tissue hypoxia despite normal oxygenation?
Answer choices
- ACarbon monoxide binding to hemoglobin and shifting the oxygen dissociation curve to the leftCorrect answer
- BCyanide inhibition of cytochrome oxidase
- CReduced inspired oxygen concentration causing low alveolar oxygen tension
- DMethemoglobin formation causing ferric iron accumulation
- ERight shifted oxygen dissociation curve from increased 2 3 BPG
- FNitric oxide formation causing vasodilation and distributive hypoxia
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