Kaplan USMLE Step 1: Blood accidentally drawn from femoral vein

Updated | 3 Min Read

If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.

Free study aid when you join the AMA

Medical students who become AMA members can select a free gift from a variety of powerful study aids for the USMLE and COMLEX-USA tests.

Supporting you today as a medical student. Protecting your future as a physician.

Learn More

A 48-year-old man with quadriplegia is admitted to the intensive care unit because of urosepsis. While trying to get a blood sample for arterial blood gas from the femoral artery, it is accidentally drawn from the femoral vein.

When compared to the arterial blood sample, the venous sample will most likely show which of the following differences?

Kaplan USMLE Step 1: Blood accidentally drawn from femoral vein answer options

 

 

 

 

 

 

 

 

 

 

 

 

The correct answer is B.

Kaplan Medical explains why

This patient had blood drawn from the femoral vein instead of the femoral artery. The arteries carry oxygenated blood to the tissues and the tissues use oxygen, so venous blood has decreased PO2 compared to arterial blood. The tissues produce CO2, which dissolves in water to form carbonic acid, and thus causes the venous blood to be slightly more acidic (decreased pH) than arterial blood.

Why the other answers are wrong

Choices A, D and E: These options can be ruled out because venous blood should have a higher PCO2 than arterial blood.

 

Did you answer the question correctly?

Sign in to get tips to remember and learn more from Kaplan Medical about why the other answer choices can be eliminated.

AMA membership has its advantages

  • Access education programs and study guides to help you thrive.
  • Distinguish yourself with AMA leadership opportunities.
  • Enjoy exclusive perks and savings.

Not a member? Become a member now.

FEATURED STORIES FOR MEDICAL STUDENTS

Hand holding a document approaches three figures on pedestals

How do residency programs rank you? Top 5 factors PDs consider

| 7 Min Read
Three rows of sticky notes

Creating a Match rank-order list? Avoid these 4 common missteps

| 6 Min Read
Glue creating a speech bubble

From day 1 to Match Day, these medical student specialty picks stick

| 5 Min Read
Device screen recording a video

On TikTok, solid health information outperforms misleading videos

| 4 Min Read