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USMLE Step 1 Questions
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Gram-positive cocci in clusters are seen in purulent fluid drained from a skin abscess. Rapid identification of these organisms will be facilitated by evaluation of the clumping of latex beads coated with which of the following?
Correct Answer: A. IgG and fibrinogen
In the question stem, the gram positive cocci in clusters are most likely Staph aureus and therefore, the answer choices must pertain to specific attributes of S. aureus. S. aureus expresses protein A which will bind to IgG and fibrinogen receptors (S. aureus clumping factor) which will bind fibrinogen (option A).
IL-1 and factor VIII (B) are produced by the body and therefore will not identify S. aureus.
Properdin and platelet factor 3 (C) are related to complement and platelet aggregation.
Prothrombin and C3b (D) are related to thrombin (coagulation cascade leading to clot formation) and complement factor part of immune response.
Transferrin is an iron carrier in the blood and plasminogen is precursor for plasmin which is an enzyme involved in fibrinolysis (clot busting).
1. QuestionCategory: Microbiology
Gram-positive cocci in clusters are seen in purulent fluid drained from a skin abscess. Rapid identification of these organisms will be facilitated by evaluation of the clumping of latex beads coated with which of the following?Correct