**Core Concept**
The infant's symptoms, including microcephaly, seizures, and psychomotor retardation, are suggestive of a congenital metabolic disorder. This condition affects the body's ability to metabolize certain compounds, leading to a buildup of toxic substances.

**Why the Correct Answer is Right**
The infant's presentation is consistent with Phenylketonuria (PKU), a genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase. This enzyme is essential for the breakdown of phenylalanine, an amino acid found in many protein-rich foods. In PKU, phenylalanine accumulates and is converted to phenylpyruvic acid, which is toxic to the brain. The musty odor of the urine is a classic sign of PKU due to the presence of phenylacetate and phenylpyruvate.

**Why Each Wrong Option is Incorrect**
**Option A:** PKU is an autosomal recessive disorder, not X-linked.
**Option B:** The disease is characterized by intellectual disability, not just psychomotor retardation.
**Option C:** The infant's MRI brain would likely show white matter abnormalities, not just a normal brain structure.

**Clinical Pearl / High-Yield Fact**
PKU is a treatable condition if diagnosed early. A newborn screening test can detect high levels of phenylalanine in the blood, allowing for prompt initiation of dietary restrictions and supplements to prevent brain damage.

**Correct Answer:** D.