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

## **Why the Correct Answer is Right**
Sucrose, also known as table sugar, is a disaccharide composed of glucose and fructose. It does not have a free aldehyde or ketone group because these groups are involved in the glycosidic bond between the glucose and fructose molecules. As a result, sucrose cannot open up its ring structure to form an aldehyde group, which is necessary for a sugar to act as a reducing agent. Therefore, sucrose is a non-reducing sugar.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Glucose is a monosaccharide with a free aldehyde group, making it a reducing sugar. It can easily open up its ring structure to form an aldehyde, which allows it to act as a reducing agent.
- **Option B:** Lactose is a disaccharide made of glucose and galactose. It has a free aldehyde group, making it capable of acting as a reducing sugar.
- **Option C:** Fructose is a monosaccharide with a ketone group. While it is a bit more complex because it typically exists in a furanose form, fructose can isomerize to glucose (an aldose) under alkaline conditions, which then can act as a reducing sugar. Directly, in its open-chain form, it can also act as a reducing sugar.

## **Clinical Pearl / High-Yield Fact**
A useful clinical pearl to remember is that reducing sugars can be detected using Benedict's test. This is particularly relevant in clinical settings for detecting glycosuria (presence of glucose in urine), which can be an indicator of diabetes mellitus.

## **Correct Answer:** . Sucrose