Which organism is most likely to be isolated from a child with bloody diarrhea that is Lac+ and glucose-fermenting?

The organism most likely to be isolated from a child with bloody diarrhea that is lactose-positive and glucose-fermenting is Escherichia coli.

Escherichia coli (E. coli) is a versatile bacterium known for its ability to ferment lactose due to the presence of the enzyme β-galactosidase, which allows it to utilize lactose as a carbon source. Specific pathogenic strains of E. coli, such as Enterohemorrhagic E. coli (EHEC), can cause bloody diarrhea and are associated with outbreaks of foodborne illness. These strains produce Shiga toxin, leading to bloody diarrhea and potential complications like hemolytic uremic syndrome (HUS).

In the context of the question, the presence of bloody diarrhea strongly suggests a pathogenic strain of E. coli, particularly EHEC, which would demonstrate both lactose fermentation and glucose fermentation. This characteristic differentiates it from other pathogens listed. For example, Shigella species do not ferment lactose and would not yield a positive result in that regard. Salmonella, while it can cause bloody diarrhea, is typically lactose-negative and does not ferment lactose. Pseudomonas aeruginosa is also lactose-negative and is not commonly associated with bloody diarrhea