Two days after eating shellfish - proteus

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Two days after eating shellfish, a 29-year-old woman develops voluminous rice-watery diarrhea. A stool culture reveals gram negative, comma-shaped rods that grow yellow colonies on thiosulfate-citrate-bile salts-sucrate (TCBS) agar. The toxin responsible for this illness acts by which of the following mechanisms?

A. ADP-ribosylation of Gi protein which results in increased levels of cAMP
B. ADP-ribosylation of Gi protein which results in decreased levels of cAMP
C. ADP-ribosylation of Gs protein which results in increased levels of cAMP
D. ADP-ribosylation of Gs protein which results in decreased levels of cAMP
E. Phosphorylation of a tyrosine kinase
F. Dephosphorylation of a tyrosine kinase

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CCCC.

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c.....

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c v. vulni

[ArchivalUser]

Choice © is the correct answer. Cholera toxin acts by ADP-ribosylating the alpha-unit of Gs protein. This prevents the G protein from hydrolyzing GTP and results in a chronically active state. cAMP increases and the net effect is the efflux of chloride and water into the intestinal lumen which results in watery diarrhea. Cholera toxin causes cAMP levels to increase not decrease (Choices B, D). (Choice A) describes the action of pertussis toxin which ADP-ribosylates Gi protein and causes an increase in cAMP. (Choices E and F) are incorrect.