## **Core Concept**
The question describes a child with microcephaly, blue eyes, fair skin, mental retardation, and a positive ferric chloride test. This combination of symptoms suggests a metabolic disorder. The ferric chloride test is used to detect phenylketones in the urine, which are indicative of phenylketonuria (PKU).

## **Why the Correct Answer is Right**
Phenylketonuria (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. This leads to the accumulation of phenylalanine and its metabolites, including phenylpyruvic acid, which can cause intellectual disability if not treated early. The symptoms described, such as microcephaly, fair skin, blue eyes, and mental retardation, are classic for untreated PKU. The positive ferric chloride test, which turns the urine green due to the presence of phenylpyruvic acid, supports this diagnosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without the specific details of Option A, it's not possible to directly refute it, but given the context, any option not aligning with the symptoms and test results indicative of PKU would be incorrect.
- **Option B:** Similarly, without specifics, any alternative diagnosis not matching the provided clinical scenario and diagnostic test result would not be correct.
- **Option C:** Assuming C is not PKU, it would not explain the positive ferric chloride test or the full spectrum of symptoms presented.
- **Option D:** If D is not the correct diagnosis of PKU, it would not align with the clinical presentation and the positive ferric chloride test.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that early diagnosis and treatment of PKU with a phenylalanine-restricted diet can significantly improve outcomes and prevent intellectual disability. The ferric chloride test is a useful screening tool for PKU, turning green in the presence of phenylketones.

## **Correct Answer:** C. Phenylketonuria.