Which of the following gives a negative Benedict's test?

INI-CET EXAM -2026

Biochemistry & Nutrition

Easy

Appeared in: INI-CET EXAM -2026

Explanation

Benedict's test identifies reducing sugars, which possess a free aldehyde or ketone group.
Sucrose is a disaccharide formed by a glycosidic bond between the anomeric carbons of glucose and fructose.
This bond formation masks the potentially free aldehyde and ketone groups of its constituent monosaccharides.
Without a free reducing group, sucrose cannot reduce the copper ions in Benedict's reagent.
Therefore, sucrose is classified as a non-reducing sugar and yields a negative result (the solution remains blue).

Option B: Glucose is a monosaccharide and a classic example of a reducing sugar because it has a free hemiacetal group that can open to form an aldehyde, which readily reacts in Benedict's test.
Option C: Fructose is a ketose monosaccharide. In the alkaline environment of the Benedict's reagent, it undergoes isomerization to form aldoses (glucose and mannose), which are reducing sugars. Thus, fructose gives a positive test.
Option D: Ascorbic acid (Vitamin C) is a potent non-carbohydrate reducing agent. Its chemical structure allows it to readily donate electrons and reduce the Cu2+ ions in the reagent, resulting in a positive test.

Clinical Relevance

Nursing practice connection: Knowing Biochemical tests for carbohydrates: Benedict's test for reducing and non-reducing sugars helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Understanding the principle of Benedict's test is crucial for interpreting older clinical data and for recognizing its limitations in modern practice.
Nurses should be aware that a positive Benedict's test for urine sugar does not definitively mean glucosuria. It could be due to other sugars (e.g., lactosuria in a breastfeeding mother) or interfering substances like high-dose Vitamin C.
This knowledge reinforces the importance of using specific tests, like the glucose oxidase method, for accurate monitoring of diabetic patients.

How to Approach the Question

First, identify the core of the question: it asks which substance gives a negative Benedict's test.
Recall the principle of Benedict's test: it is a test for reducing substances, primarily sugars with a free aldehyde or ketone group. A negative result implies the substance is non-reducing.
Systematically evaluate each option's chemical nature.
Analyze Sucrose: Recognize it as a disaccharide. Recall that in sucrose, the reducing ends of both glucose and fructose are joined in a glycosidic bond, making it non-reducing.
Analyze Glucose and Fructose: Identify them as monosaccharides, which are the classic examples of reducing sugars.
Analyze Ascorbic acid: Recall that Vitamin C is a strong reducing agent, which would also react.

Concept Tested & Keywords

Concept Tested: Biochemical tests for carbohydrates: Benedict's test for reducing and non-reducing sugars.
Stem keywords: Benedict's test, negative
Lead-in keywords: Which
Negative lead-in flag: false

Qde-AIkPq4gLlcfY3zGDc2

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