## Core Concept
The question pertains to the diagnosis of Phenylketonuria (PKU) using the ferric chloride test. PKU is a genetic disorder characterized by the inability to break down the amino acid phenylalanine (Phe), due to a deficiency in the enzyme phenylalanine hydroxylase. The ferric chloride test is a screening tool used to detect the presence of phenylketones in the urine, which are abnormal metabolites associated with PKU.

## Why the Correct Answer is Right
The ferric chloride test involves adding ferric chloride to a urine sample, which results in a color change if certain compounds, such as phenylketones, are present. In the case of PKU, the urine turns **green** due to the presence of phenylpyruvic acid, a metabolite of phenylalanine that accumulates when phenylalanine hydroxylase is deficient. This green color is a result of the reaction between ferric chloride and phenylpyruvic acid.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not correspond to the green color change associated with PKU.
- **Option B:** This option suggests another color, which is not the correct association for PKU.
- **Option D:** Similarly, this option does not match the green color produced by the ferric chloride test in the context of PKU.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the ferric chloride test can detect several abnormal metabolites in urine, each producing a different color change. For example, in PKU, the color change is **green**, in salicylate poisoning, it's **purple**, and in ketones, it's **purple** as well. This test, while not specific, serves as a useful screening tool for various metabolic disorders.

## Correct Answer: .