A 3-year-old child is evaluated by a pediatrician for poor growth despite excessive food intake. The mother repos that the child's stools are bulky, foul-smelling, and difficult to flush because they float. Determination of which of the following would most likely be diagnostic in this case?

This is one of three common presentations for cystic fibrosis. The two other common presentations are meconium ileus in infancy and multiple respiratory tract infections, often in later childhood. Cystic fibrosis was the first genetic disease for which a specific lab test was developed: the sweat test. This test is still used diagnostically; in its modern form, in which pilocarpine is used to stimulate sweat secretion. The basic underlying defect in cystic fibrosis is an abnormality of the chloride channel, but both the Na+ and Cl- content of the sweat of affected children is increased compared to normal. The Na+ content of CSF, serum, urine, and whole blood is normal in children with cystic fibrosis.