In an experiment,a researcher raises mice under completely germfree conditions so that colonization of their intestine with commensal bacteria cannot occur.In comparison with a conventionally raised animal,the concentration of which of the following would be expected to be increased in the colonic contents of such mice?

A child is brought to a gastroenterologist because of persistent diarrhea,particularly after fatty meals.An extensive workup eventually reveals that the child lacks the ability to synthesize peptide YY.The increased stool volume seen in the child is probably ascribable primarily to abnormally high active secretion of which of the following?

A college student decides to enter the Peace Corps after graduation and is assigned to teach English in sub-Saharan Africa,where the ambient temperature averages 100 $\degree$ F.During the first few weeks of his stay,he notes copious sweating and mild constipation.His gastrointestinal symptoms are probably due in part to upregulated expression of which of the following transport proteins?