**Core Concept**
The combination of microcephaly, blue eyes, fair skin, mental retardation, and a positive ferric chloride test points towards a disorder of amino acid metabolism. This condition is characterized by the accumulation of phenylalanine and its metabolites in the body due to a deficiency in the enzyme phenylalanine hydroxylase.

**Why the Correct Answer is Right**
The positive ferric chloride test is indicative of the presence of phenylketones in the urine, which is a hallmark of phenylketonuria (PKU). PKU is caused by a mutation in the PAH gene that encodes for the phenylalanine hydroxylase enzyme. This enzyme is responsible for converting phenylalanine to tyrosine. In PKU, the deficiency of this enzyme leads to the accumulation of phenylalanine, which is toxic to the developing brain, resulting in microcephaly, mental retardation, and other neurological abnormalities. The blue eyes and fair skin are non-specific findings but are often associated with fair-skinned individuals.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.
* **Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
PKU is a classic example of a genetic disorder that can be prevented with early diagnosis and treatment. Newborn screening for PKU is widely available, and early initiation of dietary restrictions can prevent the development of neurological symptoms.

**Correct Answer:** D.