USMLE-Rx Step 1 Practice Q's

USMLE-Rx Step 1 Qmax Challenge #4487

Check out today’s Step 1 Qmax Question Challenge.

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USMLE-Rx Step 1 Qmax Challenge #4487The image depicts the cell wall of a gram-negative bacterium.

What structure is responsible for the pathogenicity of the organism?

A. A
B. B
C. C
D. D
E. E


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5 replies »

  1. The correct answer is A. The pathogenicity of a gram-negative bacterium is largely due to structures in the outer membrane. This membrane consists of a lipopolysaccharide (diagrammed in the image) and various proteins such as endotoxins. Endotoxins, specifically lipid A, can activate the complement cascade, the coagulation cascade, and macrophages. The outer membrane proteins have enormous genetic and antigenic variability.

    B is not correct. The periplasmic space is immediately beneath the outer membrane and is adjacent to the peptidoglycan, which forms the major cell wall constituent.

    C is not correct. The inner cytoplasmic membrane contains penicillin-binding proteins.

    D is not correct. Porins, or hydrophilic channels, trigger host cell endocytosis of the organism, an important step in early inflammation.

    E is not correct. Choice E represents the peptidoglycan layer. Inhibition of peptidoglycan synthesis is the mechanism of action for the penicillins and cephalosporins.


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