A 3-month-old male infant presents with chronic diarrhea, failure to thrive, and hypoproteinemia despite adequate milk intake. Laboratory studies show normal pancreatic enzyme levels in serum and normal stool chymotrypsin. Genetic testing reveals a homozygous loss-of-function mutation in the gene encoding the brush-border enzyme marked **B** in the diagram. Which of the following best explains the pathophysiology of this infant's presentation?