An isolate of E. coli recovered from the stool of a patient with severe bloody diarrhea should be tested for which sugar?

Testing the isolate of E. coli from a patient with severe bloody diarrhea for sorbitol fermentation is particularly relevant because certain pathogenic strains of E. coli, notably the O157:H7 serotype, do not ferment sorbitol. In a clinical setting, if the E. coli isolate can ferment sorbitol, it is less likely to be the pathogenic O157:H7 strain, which is often associated with hemorrhagic colitis and can cause severe complications like hemolytic uremic syndrome.

The other sugars listed—mannitol, raffinose, and sucrose—are less specific to identifying this pathogenic strain. For example, while some bacteria can ferment mannitol, it's not a reliable indicator for distinguishing pathogenic E. coli subsets. Raffinose and sucrose are likewise used in various fermentation tests but do not have the same clinical significance in the context of diagnosing infections caused by enterohemorrhagic E. coli. Thus, the focus on sorbitol fermentation provides a direct test that can aid in the identification of the potentially dangerous strain present in cases of severe bloody diarrhea.