**Core Concept**
Mental retardation, now referred to as intellectual disability, is a condition characterized by significant limitations in both intellectual functioning and adaptive behavior, which covers a range of everyday social and practical skills. Intellectual disability is generally categorized into different types based on etiology and severity.

**Why the Correct Answer is Right**
The most common type of intellectual disability is Down syndrome, also known as trisomy 21. It occurs due to the presence of an extra copy of chromosome 21, leading to an overdose of genes that code for proteins involved in brain development. This results in significant impairments in cognitive and adaptive skills. The extra chromosome 21 disrupts normal brain development, affecting areas responsible for learning and memory.

**Why Each Wrong Option is Incorrect**
**Option A:** Fragile X syndrome is another common cause of intellectual disability, but it is less common than Down syndrome. Fragile X syndrome is caused by an expansion of the CGG repeat in the FMR1 gene on the X chromosome.
**Option B:** Phenylketonuria (PKU) is a genetic disorder that can cause intellectual disability if left untreated. However, it is relatively rare compared to Down syndrome.
**Option C:** Fetal alcohol syndrome is a condition that can cause intellectual disability and other birth defects in children whose mothers drink alcohol during pregnancy. While it is a significant concern, it is not the most common type of intellectual disability.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that intellectual disability is a heterogeneous condition with varying etiologies and severities. A thorough diagnostic evaluation, including genetic testing, is necessary to determine the underlying cause of intellectual disability in an individual.

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