**Core Concept**
Mental retardation, now more appropriately termed intellectual disability, is a neurodevelopmental disorder characterized by significant impairments in cognitive functioning and adaptive behavior. The condition is often associated with genetic, environmental, or metabolic factors that affect brain development.

**Why the Correct Answer is Right**
The most common type of intellectual disability is Down syndrome, which accounts for approximately 20-30% of all cases. This condition is caused by the presence of an extra copy of chromosome 21 (trisomy 21), leading to an overexpression of genes involved in brain development and function. The resulting cognitive and adaptive impairments are often accompanied by characteristic physical features, such as a flat face, small ears, and an enlarged tongue.

**Why Each Wrong Option is Incorrect**
**Option A:** Fragile X syndrome is a genetic disorder that is the most common cause of inherited intellectual disability, but it is not the most common type overall.
**Option B:** Fetal alcohol syndrome is a preventable cause of intellectual disability, but it is not a distinct type of intellectual disability.
**Option C:** Phenylketonuria (PKU) is a metabolic disorder that can cause intellectual disability if left untreated, but it is not a type of intellectual disability in itself.

**Clinical Pearl / High-Yield Fact**
Down syndrome is often associated with an increased risk of certain medical conditions, such as heart defects, vision and hearing impairments, and obesity.

**Correct Answer:** D. Down syndrome.