Which one among the following gives a negative Benedict's test?

HP CHO -9 October 2022

Biochemistry & Nutrition

Easy

Appeared in: HP CHO -9 October 2022

Explanation

Sucrose is classified as a non-reducing sugar.
It is a disaccharide composed of a glucose unit and a fructose unit.
The linkage between them (a glycosidic bond) involves the anomeric carbon of glucose (C1) and the anomeric carbon of fructose (C2).
These anomeric carbons contain the potentially reactive aldehyde and ketone groups. Since they are locked in the bond, there is no free hemiacetal group available to open up and react.
Without a free aldehyde or ketone group, sucrose cannot reduce the copper(II) ions in Benedict's reagent, resulting in a negative test (the solution remains blue).

Option A: Fructose is a reducing sugar. Although it is a ketose (contains a ketone group), in the alkaline conditions of the Benedict's test, it undergoes tautomerization (rearrangement) to form an aldose (like glucose), which has a free aldehyde group that gives a positive result.
Option C: Ascorbate (Vitamin C) is a powerful reducing agent. It is not a carbohydrate, but it readily donates electrons and reduces the copper(II) ions in Benedict's reagent, causing a positive test. In clinical settings, this is often considered a false-positive result when testing for urine sugar.
Option D: Glucose is the classic example of a reducing sugar. It is an aldose monosaccharide with a free hemiacetal group that can open to expose an aldehyde group, which readily reacts with Benedict's reagent to give a strong positive test.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Identification of reducing and non-reducing sugars using Benedict's test as background academic context rather than a clinical decision trigger.
Historically, Benedict's test was a cornerstone in clinical chemistry for detecting glycosuria (sugar in the urine), a common indicator of diabetes mellitus.
A key nursing responsibility is understanding the limitations of diagnostic tests. Benedict's test is non-specific; a positive result can be caused by glucose, other sugars (lactose, fructose), or non-sugar reducing agents like ascorbic acid (Vitamin C) and certain drugs, leading to false positives.
Modern practice has largely replaced Benedict's test with highly specific glucose oxidase enzyme strips, which only react with glucose, providing more accurate results for diabetes monitoring.

How to Approach the Question

First, identify the core concept of the question, which is the Benedict's test.
Recall the principle of this test: it is used to detect the presence of reducing substances, primarily reducing sugars, which possess a free aldehyde or ketone group.
Analyze each option based on its chemical structure and properties.
Evaluate Glucose and Fructose: Both are monosaccharides and are known to be reducing sugars.
Evaluate Ascorbate: Recall that Vitamin C is a strong reducing agent and can give a positive test.
Evaluate Sucrose: Recall that it is a disaccharide formed by a bond between the reducing ends of glucose and fructose, making it a non-reducing sugar.

Concept Tested & Keywords

Concept Tested: Identification of reducing and non-reducing sugars using Benedict's test.
Stem keywords: Benedict's test, negative
Lead-in keywords: Which one
Negative lead-in flag: false

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E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 1 (pp 26-370 of 370) pp. 73-75, 188-190