**Core Concept:** Metabolic disorders in infants can present with a constellation of neurological symptoms, including mental retardation, regression of motor skills, feeding problems, impaired head control, and macrocephaly. MRI can help identify the underlying pathology.

**Why the Correct Answer is Right:** The correct answer, **C. Lissencephaly**, is a malformation of the brain characterized by a smooth, featureless appearance on MRI scans due to the absence of grooves and fissures on the cerebral cortex. This can lead to the symptoms mentioned, as it affects brain development and function.

**Why Each Wrong Option is Incorrect:**

A. **Pyruvate Dehydrogenase Complex (PDC) Deficiency (E1)**: Although this disorder can cause neurological symptoms, it typically presents with seizures, hypotonia, developmental delay, and feeding difficulties. Head control is usually preserved, which makes it less likely than Lissencephaly.

B. **Glycogen Storage Disease Type I (GSD I)**: This disorder presents with hepatomegaly, macrocephaly, and hypotonia, which is different from the described symptoms of mental retardation, macrocephaly, and motor regression in Lissencephaly.

D. **Krabbe Disease**: This disorder presents with macrocephaly, seizures, and spasticity, which is different from the described symptoms in Lissencephaly.

**Clinical Pearl:** Lissencephaly is a crucial differential to consider when assessing infants with mental retardation, macrocephaly, and motor regression, as it can lead to prompt diagnosis and appropriate management. MRI is essential in differentiating Lissencephaly from other disorders presenting with similar symptoms.