Final answer:
Lactase denaturation is caused by extreme pH changes in the intestines, leading to the inability of the enzyme to digest lactose, which is characteristic of lactose intolerance.
Step-by-step explanation:
Lactose intolerance occurs when lactase, the enzyme responsible for digesting the milk sugar lactose, is deficient or not functioning properly. When extreme pH changes in the intestines cause lactase to no longer function, the phenomenon that has occurred to lactase is denaturation. Denaturation involves the structural alteration of a protein, rendering it unable to perform its biological function, in this case, the digestion of lactose. Denaturation is often irreversible and can result from factors such as changes in pH levels or temperature.