## **Core Concept**
The question tests understanding of chemical tests used to detect specific functional groups in biological molecules, particularly peptides and proteins. The tests mentioned involve reactions with peptides or proteins to identify certain characteristics. An intact peptide bond is crucial for some of these tests.

## **Why the Correct Answer is Right**
The correct answer, **Biuret test**, requires an intact peptide bond. The Biuret test is a chemical test used for detecting the presence of peptide bonds. In the Biuret test, copper(II) sulfate reacts with peptides or proteins that have at least three peptide bonds, resulting in a purple-colored complex. This reaction is specific to peptide bonds, making it a useful test for proteins. The peptide bond acts as a ligand to the copper ion in an alkaline environment, leading to the color change.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **Ninhydrin test** reacts with free amino groups (not necessarily peptide bonds) to produce a purple color. This test detects amino acids, not requiring an intact peptide bond.
- **Option B:** The **Bradford assay** is a test used to measure protein concentration. It does not specifically require an intact peptide bond but rather the presence of proteins or large peptides. The assay is based on the binding of Coomassie Brilliant Blue G-250 dye to proteins.
- **Option D:** The **Ellman's test** (or Ellman reagent) is used to quantify thiol (sulfhydryl) groups, which are present in cysteine residues of proteins. This test does not require an intact peptide bond.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **Biuret test** is often used in clinical settings to detect proteins in urine (proteinuria), which can be an indicator of kidney damage. The test's reliance on peptide bonds makes it specific for proteins and not for free amino acids.

## **Correct Answer:** C. Biuret test