**Core Concept**
Mental retardation, now referred to as Intellectual Disability (ID), is a neurodevelopmental disorder characterized by significant impairments in intellectual functioning and adaptive behaviors. It is a complex condition with multiple etiologies, including genetic, environmental, and metabolic factors.

**Why the Correct Answer is Right**
Intellectual Disability is diagnosed based on a significant deficit in cognitive functioning, as measured by IQ scores below 70. Adaptive behaviors, such as communication, social skills, and daily living skills, are also impaired. The severity of ID is categorized into four levels: mild, moderate, severe, and profound. The etiology of ID can be genetic, chromosomal, or due to prenatal, perinatal, or postnatal factors.

**Why Each Wrong Option is Incorrect**
**Option A:** Mental retardation is not the preferred term due to its stigmatizing nature.
**Option B:** While Down syndrome is a cause of Intellectual Disability, it is not the only cause.
**Option C:** Intellectual Disability is not solely due to environmental factors, as genetic and prenatal factors also play a significant role.

**Clinical Pearl / High-Yield Fact**
Intellectual Disability is a lifelong condition that requires early diagnosis and intervention for optimal management and support. A comprehensive evaluation by a multidisciplinary team, including psychologists, geneticists, and pediatricians, is essential for accurate diagnosis and development of a personalized treatment plan.

**Correct Answer: D.**