Vignette: A 2-year-old boy is brought to the pediatrician by his parents, who report persistent diarrhea, abdominal bloating, and failure to thrive. They also report noticing an unusual sweet smell to his breath. His symptoms seem to worsen after consuming milk-based products. His stool is tested and found to be positive for reducing substances. Which of the following is the most likely deficiency in this patient?
This patient's symptoms of diarrhea, abdominal bloating, and failure to thrive, which worsen after consuming milk-based products, in conjunction with the presence of reducing substances in the stool, suggest lactose intolerance due to lactase deficiency. Lactase is an enzyme found in the brush border of the small intestine that breaks down lactose into glucose and galactose. Without lactase, lactose remains in the gut, drawing water into the lumen osmotically and leading to diarrhea. Bacteria in the gut can ferment the undigested lactose, leading to the production of gas (causing bloating) and the characteristic sweet-smelling breath. The failure to thrive can be attributed to the resulting malabsorption. Reducing substances in the stool confirm the presence of undigested lactose. Other listed options are other digestive enzymes but their deficiency does not result in these symptoms.