Mental retardation is defined if IQ is below –

**Core Concept**
Mental retardation, now more commonly referred to as intellectual disability, is a neurodevelopmental disorder characterized by cognitive impairments and limitations in adaptive behavior. The diagnosis is primarily based on the individual's intelligence quotient (IQ), which is a statistical measure derived from standardized intelligence tests.

**Why the Correct Answer is Right**
The World Health Organization (WHO) and the American Association on Intellectual and Developmental Disabilities (AAIDD) define intellectual disability as an IQ score below 70, which is more than 2 standard deviations below the mean on an appropriately standardized intelligence test. This definition accounts for the individual's adaptive behavior, including their ability to perform daily tasks, communicate effectively, and understand social norms. The IQ threshold is not the sole criterion for diagnosis, but it serves as a useful indicator of cognitive impairment.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because it does not accurately reflect the current definition of intellectual disability. The IQ threshold is a key component, but it is not the sole criterion for diagnosis.

**Option B:**
This option is incorrect because it is too low. An IQ score below 40 is associated with severe intellectual disability, but it is not the threshold for the diagnosis.

**Option C:**
This option is incorrect because it is too high. An IQ score above 85 is generally considered to be within the average range, and it does not meet the criteria for intellectual disability.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that intellectual disability is a heterogeneous condition, and its diagnosis should be based on a comprehensive evaluation of the individual's cognitive, adaptive, and behavioral functioning. A thorough assessment by a multidisciplinary team, including psychologists, psychiatrists, and other healthcare professionals, is necessary to ensure an accurate diagnosis and to develop an effective treatment plan.

**Correct Answer:** D.