A child has microcephaly, blue eyes, fair skin, and mental retardation; ferric chloride test is positive. What is the likely diagnosis?
**Core Concept**
The patient's symptoms suggest a congenital disorder affecting the metabolism of phenylalanine, an essential amino acid. This disorder leads to an accumulation of toxic metabolites, causing neurological damage and other systemic effects.
**Why the Correct Answer is Right**
The patient's symptoms, including microcephaly, mental retardation, and a positive ferric chloride test, are consistent with Phenylketonuria (PKU). PKU is caused by a deficiency of the enzyme phenylalanine hydroxylase, which normally converts phenylalanine to tyrosine. Without this enzyme, phenylalanine accumulates and is converted to phenylpyruvic acid, which is detected by the ferric chloride test. This accumulation of toxic metabolites leads to neurological damage and other systemic effects.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not match the patient's symptoms or the results of the ferric chloride test.
**Option B:** This option is incorrect because it is a different congenital disorder that affects the metabolism of galactose, not phenylalanine.
**Option C:** This option is incorrect because it is a different congenital disorder that affects the metabolism of tyrosine, not phenylalanine.
**Clinical Pearl / High-Yield Fact**
PKU is a classic example of a genetic disorder that can be prevented with early diagnosis and treatment, highlighting the importance of newborn screening programs.
**Correct Answer: C. Phenylketonuria (PKU)**