## **Core Concept**
The question describes a case of malabsorption related to the inability to digest a component of milk, leading to symptoms like abdominal distention, flatulence, and diarrhea. This scenario points towards a deficiency of an enzyme needed for the digestion of lactose, a sugar found in milk. The enzyme responsible for lactose digestion is located on the intestinal brush border membrane.

## **Why the Correct Answer is Right**
The correct answer, **Lactase**, is the enzyme that breaks down lactose into glucose and galactose, which can then be absorbed by the intestine. A deficiency in lactase leads to lactose intolerance, where undigested lactose is fermented by colonic bacteria, producing gas and causing the symptoms described. Lactase is specifically associated with the intestinal brush border membrane, where it performs its function.

## **Why Each Wrong Option is Incorrect**
- **Option A: Sucrase** - This enzyme is involved in the breakdown of sucrose into glucose and fructose. While it is also a part of the intestinal brush border membrane, its deficiency does not lead to the symptoms described after milk consumption.
- **Option B: Isomaltase** - This enzyme breaks down certain complex carbohydrates. Its deficiency can lead to problems with carbohydrate digestion but is not directly related to the digestion of milk sugars.
- **Option D: Alpha-glucosidase** - This is a broader category of enzymes involved in carbohydrate digestion. While related to carbohydrate metabolism, it is not specifically implicated in lactose digestion.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that lactose intolerance is common in individuals of certain ethnic backgrounds and can be managed by avoiding lactose-containing products or by taking lactase enzyme supplements. This condition illustrates the importance of brush border enzymes in nutrient absorption and the clinical consequences of their deficiency.

## **Correct Answer: C. Lactase**