## **Core Concept**
Reducing sugars are those that can donate electrons to another molecule and, in the process, reduce that molecule. This capability is typically due to the presence of an aldehyde group in their structure. Common reducing sugars include glucose, fructose, and lactose.

## **Why the Correct Answer is Right**
The correct answer, , is a reducing sugar because it has an aldehyde group in its structure. This aldehyde group can open up to form an aldehyde functional group, allowing it to act as a reducing agent. Specifically, in its open-chain form, glucose has an aldehyde group at one end, making it capable of reducing certain substances, such as Benedict's solution, which is a test for reducing sugars.

## **Why Each Wrong Option is Incorrect**
- **Option A:** is not a reducing sugar because it does not have a free aldehyde or ketone group. Its structure does not allow it to easily open up and form an aldehyde group necessary for reducing properties.
- **Option B:** is not a reducing sugar for similar reasons; its structure lacks the necessary functional group to act as a reducing agent.
- **Option D:** is not a reducing sugar; it is a disaccharide made of two glucose molecules bonded in a way that does not allow for the formation of a free aldehyde or ketone group.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Benedict's test can be used to detect reducing sugars in urine. A positive test may indicate diabetes mellitus or other conditions affecting carbohydrate metabolism. Remembering that glucose is a primary reducing sugar tested in clinical settings can help in quickly identifying conditions associated with abnormal glucose levels.

## **Correct Answer:** .