**Core Concept**
Cretinism is a preventable cause of mental retardation, resulting from untreated congenital hypothyroidism. This condition occurs when the thyroid gland fails to produce sufficient thyroid hormone, leading to developmental delays and intellectual disabilities.

**Why the Correct Answer is Right**
Cretinism is a preventable cause of mental retardation due to the availability of thyroid hormone replacement therapy. If diagnosed and treated early in life, individuals with congenital hypothyroidism can develop normally and avoid intellectual disabilities. The key enzyme involved in thyroid hormone synthesis is thyroid peroxidase, which is essential for converting iodine into active thyroid hormones. Untreated congenital hypothyroidism can lead to severe cognitive impairment, characteristic physical features, and growth retardation.

**Why Each Wrong Option is Incorrect**
**Option A:** Downs syndrome is a genetic disorder caused by an extra copy of chromosome 21, leading to intellectual disability and characteristic physical features. While it is a significant cause of mental retardation, it is not preventable through medical intervention.

**Option B:** Phenylketonuria (PKU) is a genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase, leading to the accumulation of phenylalanine in the body. If left untreated, PKU can cause intellectual disability, but it is preventable through dietary restrictions and enzyme replacement therapy.

**Option D:** Cerebral palsy is a group of neurological disorders that affect movement, muscle tone, and coordination. It is not a preventable cause of mental retardation, as it is often the result of prenatal or perinatal brain injury.

**Clinical Pearl / High-Yield Fact**
Untreated congenital hypothyroidism can lead to severe intellectual disability and characteristic physical features, including a broad chest, short stature, and delayed speech development. Early diagnosis and treatment with thyroid hormone replacement therapy can prevent these complications and ensure normal development.

**✓ Correct Answer: C. Cretinism**