**Core Concept**
Cretinism is a preventable cause of mental retardation due to congenital hypothyroidism, which can be detected and treated with thyroid hormone replacement therapy. Early detection and treatment can prevent the development of intellectual disability in affected children.

**Why the Correct Answer is Right**
Cretinism is caused by congenital hypothyroidism, which results from a deficiency of thyroid hormones essential for brain development and maturation. Untreated congenital hypothyroidism can lead to intellectual disability, growth retardation, and other developmental problems. Screening for congenital hypothyroidism at birth allows for early detection and initiation of thyroid hormone replacement therapy, preventing the development of mental retardation.

**Why Each Wrong Option is Incorrect**
**Option A:** Downs syndrome is a genetic disorder caused by an extra copy of chromosome 21 and is not preventable.
**Option B:** Phenylketonuria is a genetic disorder that affects the metabolism of the amino acid phenylalanine and is not preventable.
**Option D:** Cerebral palsy is a group of neurological disorders that affect movement and muscle coordination, and is not preventable.

**Clinical Pearl / High-Yield Fact**
Early detection and treatment of congenital hypothyroidism through newborn screening is a critical public health initiative that has significantly reduced the incidence of mental retardation due to cretinism.

**✓ Correct Answer: C. Cretinism**