**Core Concept**
The question tests the understanding of carbohydrate digestion in the gastrointestinal tract (GIT), specifically the breakdown of disaccharides by enzymes such as **lactase**, **sucrase**, and **maltase**.

**Why the Correct Answer is Right**
The correct disaccharide is not broken down in the GIT due to the lack of a specific enzyme. For example, individuals with **lactose intolerance** cannot digest **lactose** because they have low levels of **lactase**.

**Why Each Wrong Option is Incorrect**
**Option A:** Lactose is broken down by lactase.
**Option B:** Sucrose is broken down by sucrase.
**Option C:** Maltose is broken down by maltase.

**Clinical Pearl / High-Yield Fact**
Remember that **lactose intolerance** leads to undigested lactose in the colon, causing bloating and diarrhea.

**Correct Answer:** D. Lactose