**Core Concept**
Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This condition leads to intellectual disability, delayed speech, and characteristic physical features.

**Why the Correct Answer is Right**
Down syndrome is the most common hereditary cause of intellectual disability due to the presence of an extra copy of chromosome 21. This extra genetic material disrupts normal brain development and function, leading to various cognitive and physical impairments. The condition is caused by nondisjunction during meiosis, resulting in an abnormal number of chromosomes. Individuals with Down syndrome often exhibit distinct physical features, such as a flat face, short neck, and protruding tongue.

**Why Each Wrong Option is Incorrect**
**Option A:** Fragile X syndrome is another genetic cause of intellectual disability but is less common than Down syndrome.
**Option B:** Prader-Willi syndrome is a genetic disorder that affects growth and development but is not the most common hereditary cause of intellectual disability.
**Option C:** Phenylketonuria (PKU) is a metabolic disorder that can lead to intellectual disability if left untreated but is not a hereditary cause.

**Clinical Pearl / High-Yield Fact**
Down syndrome is the most common chromosomal disorder, occurring in approximately 1 in 700 births. Individuals with Down syndrome are also at increased risk of developing leukemia and Alzheimer's disease.

**Correct Answer: A. Down syndrome**