USMLE Step 1 Practice Exam 2025 – 400 Free Practice Questions to Pass the Exam

Lactose intolerance occurs primarily due to a decrease in lactase enzyme production in the small intestine. Lactase is responsible for breaking down lactose, the sugar found in milk and dairy products, into glucose and galactose, which can then be absorbed into the bloodstream. When there is a downregulation, or reduction, in the synthesis of lactase, lactose is not adequately digested, leading to symptoms such as bloating, diarrhea, and abdominal pain upon the ingestion of lactose-containing foods.

In some individuals, particularly adults, lactase levels naturally decrease after weaning, as humans are not typically exposed to lactose after early childhood. This downregulation can be influenced by genetic factors and can result in varying degrees of lactose intolerance among different populations.

Other options do not align with the physiological basis of lactose intolerance. Excessive lactase production and upregulation would result in normal lactose digestion, while intracellular calcium accumulation does not directly relate to lactase function or lactose digestion.