If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 3 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.
This month’s stumper
A previously healthy 34-year-old man is admitted to a hospital’s trauma unit after sustaining multiple compound fractures in a motor vehicle accident. Shortly after arrival he is noted to be pale and hypotensive with a hematocrit of 21 percent. He is taken to surgery, and during the procedure there is further extensive blood loss. For this he is transfused with a total of four units of packed red blood cells. His surgery is otherwise uncomplicated and afterward he is transferred to the step-down unit for observation.
Initially he does well and is readily conversant and in no distress. Later that evening he becomes acutely dyspneic over the course of approximately one hour. Oxygen saturation drops to 84 percent while breathing room air, his respiratory rate is 28 per minute, and his pulse increases to 140 per minute. Lung examination reveals diffuse fine rales and increased work of breathing. There are no rashes or evidence of bleeding from his surgical wounds. A chest radiograph reveals diffuse, patchy infiltrates in a perihilar “bat-wing” pattern.
The laboratory finding, if present, that would confirm the diagnosis of transfusion-associated acute lung injury (TRALI) is:
A. Antigranulocyte antibodies in the blood donor.
B. Extensive pulmonary ground-glass infiltrates on CT scan of the thorax.
C. High levels of serum complement in the blood donor.
D. Low or absent levels of IgA in the transfusion recipient.
E. Positive direct Coombs test in the transfusion recipient.
The correct answer is A.
Kaplan Medical explains why
TRALI is characterized by acute development of hypoxemia and pulmonary infiltrates within one to 24 hours of blood transfusion. Although other factors are involved, the reaction is usually brought about by antibodies in donor blood against the recipient’ s granulocytes.
These antibodies cause complement-mediated lysis of these cells within the pulmonary vascular bed, resulting in extensive release of their proteolytic enzymes and oxygen free radicals. The ensuing lung damage resembles many other acute lung injuries and can progress to a clinically indistinguishable acute respiratory distress syndrome picture. The diagnosis is made by noting the relationship of the event to a blood transfusion and is confirmed by finding the antibodies mentioned in donor blood.
Why the other answers are wrong
Choice B: Although in TRALI one would expect to see extensive pulmonary ground-glass infiltrates on CT scan, this is a nonspecific finding. It can be present in a variety of syndromes ranging from pulmonary edema to viral pneumonitis and does not confirm any single diagnosis.
Choice C: Lysis of transfusion-recipient granulocytes is mediated by complement activation by donor antibodies, but donor levels of complement do not have an effect without such antibodies. A finding of low levels of serum complement in the recipient suggests an ongoing inflammatory reaction that consumes complement but is also a nonspecific finding.
Choice D: Congenital IgA deficiency is a common syndrome, and low or absent levels of IgA in the transfusion recipient confirms that diagnosis. The transfusion reaction seen in that syndrome is anaphylactoid and would not show pulmonary infiltrates. Patients typically develop urticaria and hypotension related to systemic vasodilatation.
Choice E: In hemolytic reactions caused by antibodies against red blood cells, a direct Coombs test is positive. In this test, IgM directed against complement protein C3d or the Fc portion of IgG is added to the subject’s blood. The test is positive if red cell agglutination occurs. This patient is not having a hemolytic reaction, and so the Coombs test would not be useful.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
The AMA and Kaplan have teamed up to support you in reaching your goal of passing the USMLE® or COMLEX-USA®. If you're looking for additional resources, Kaplan provides free access to tools for pre-clinical studies, including Kaplan’s Lecture Notes series, Integrated Vignettes, Shelf Prep and more.
