USMLE Step 1 Question (Hematology)
Understanding ABO classification
The following is a high-yield USMLE Step 1 hematology question.
It is followed by a deep dive into each answer choice—the answer can be found at the bottom of the post.
Good luck!
QUESTION:
An 18-year-old female is visiting a blood donation clinic to give blood for the first time. Prescreening shows that she has no antigens on the surface of her red blood cells. Based on this information, she can safely donate blood to which of the following types?
A. Type A
B. Type A and B
C. Type A, B, and AB
D. Type AB
E. Type A, B, and O
F. Type A, B, AB, and O
To see the answer, scroll to the bottom of the page. See the DEEP-DIVE below if you’re unsure or want to learn more about the answer choices.
DEEP-DIVE:
The ABO blood group system is a classification of human blood based on the presence or absence of antigens on the surface of red blood cells. There are two main antigens in the ABO system: Antigen A and Antigen B. Based on these antigens, blood can be categorized into four types:
Type A: Has A antigens on the surface of the red blood cells and B antibodies in the plasma.
Type B: Has B antigens on the surface of the red blood cells and A antibodies in the plasma.
Type AB: Has both A and B antigens on the surface of the red blood cells and no A or B antibodies in the plasma. This type can receive blood from any ABO blood type, making it the universal recipient.
Type O: Has no antigens on the surface of the red blood cells but has both A and B antibodies in the plasma. Since it has no A or B antigens, it can donate blood to any ABO blood type, making it the universal donor. The patient in question is said to have no antigens on the surface of her RBCs - thus, she is blood type O.
A. TYPE A
This is incorrect because individuals with blood type A can only donate to other A types and AB types due to the presence of A antigens.
B. TYPE A and B
This is incorrect because individuals with blood type O can donate to all ABO blood types, not just A and B.
C. TYPE A, B, and AB
While technically correct, this isn’t the best overall answer choice as it lacks completeness.
D. TYPE AB
This is incorrect because while type AB individuals can receive blood from any type due to being universal recipients, type O blood can be donated to individuals with any ABO blood type, including O itself.
E. TYPE A, B, & O
This is incorrect because it excludes type AB, which can also receive blood from type O donors.
F. TYPE A, B, AB, & O
This is the correct answer because blood type O can donate to anyone since it lacks A and B antigens on the surface of its red blood cells. The absence of these antigens means that type O blood will not trigger an immune response when transfused into a recipient with any other blood type (A, B, AB, or O).
VERDICT: Blood type O is the ‘universal donor’ because it lacks A and B antigens on the surface of its RBCs. The absence of these antigens means that type O blood will not trigger an immune response when transfused into a recipient with any other blood type (A, B, AB, or O). The presence of anti-A and anti-B antibodies in the plasma of type O blood does not typically cause a problem in the recipient because the amount of plasma transferred during a standard red blood cell transfusion is relatively small and diluted by the recipient's own plasma.
FINAL ANSWER: F. TYPE A, B, AB, & O
