Final answer:
Lactase is the enzyme present in the intestinal juice of normal young horses that is necessary for the cleavage of lactose. It breaks down lactose into glucose and galactose. Insufficient amounts of lactase can result in lactose intolerance.
Step-by-step explanation:
The enzyme present in the intestinal juice of normal young horses that is necessary for the cleavage of lactose is lactase. Lactase is responsible for breaking down lactose, a disaccharide found in milk, into glucose and galactose. Without sufficient amounts of lactase, lactose cannot be digested properly and can lead to lactose intolerance.