**Core Concept**
Lactose intolerance is a condition characterized by the inability to digest lactose, a disaccharide sugar found in milk, due to a deficiency in lactase, the enzyme responsible for its hydrolysis.

**Why the Correct Answer is Right**
Lactose intolerance occurs when the small intestine does not produce enough lactase enzyme, which breaks down lactose into glucose and galactose. This deficiency is often due to a congenital condition or acquired lactase non-persistence, where the body stops producing lactase enzyme after weaning. As a result, undigested lactose reaches the large intestine, where it is fermented by bacteria, producing gas, bloating, and diarrhea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to lactose intolerance, as it is not a known cause.
**Option B:** While enzyme deficiency is indeed the cause of lactose intolerance, this option is too vague and does not specify the enzyme involved.
**Option C:** This option is incorrect, as lactose intolerance is not directly related to pancreatic insufficiency, which affects the production of digestive enzymes in the pancreas.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that lactose intolerance is not a true allergy, and individuals with this condition can still consume small amounts of lactose without significant symptoms.

**Correct Answer: D. Lactase enzyme deficiency.**