## **Core Concept**
The Biuret test is a chemical test used for detecting the presence of peptide bonds, which are the bonds that connect amino acids to form proteins. This test is based on the reaction of peptides or proteins with copper(II) sulfate in an alkaline medium, resulting in a purple-colored complex.

## **Why the Correct Answer is Right**
The Biuret test is specifically used for detecting proteins that have more than two peptide bonds. In the presence of peptides or proteins, copper ions in the alkaline solution form a purple-colored complex. This reaction is due to the coordination of copper ions with the nitrogen atoms of the peptide bonds. The test is commonly used in biochemistry to estimate protein concentrations.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while the Biuret test does involve a reaction with copper, it's not specifically testing for the presence of copper itself but rather the presence of peptide bonds.
- **Option B:** This option is incorrect because the Biuret test is not used for detecting reducing sugars; tests like Benedict's test are used for that purpose.
- **Option C:** This option might seem plausible since the Biuret test does involve an alkaline medium and copper(II) sulfate, but it's not testing for the presence of these reagents themselves.
- **Option D:** This option is incorrect because, although nucleic acids are important biomolecules, the Biuret test is not used for their detection.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the Biuret test is positive for proteins with more than two peptide bonds, making it useful for detecting proteins in biological samples. A classic clinical correlation is its use in assessing liver function by measuring serum protein levels.

## **Correct Answer:** B. Proteins.