## **Core Concept**
Mental retardation, now more appropriately referred to as intellectual disability, is a condition characterized by significant limitations in both intellectual functioning and in adaptive behavior, which covers many everyday social and practical skills. This condition originates before the age of 18. The intellectual functioning is quantitatively assessed by the Intelligence Quotient (IQ) score.

## **Why the Correct Answer is Right**
The correct answer, , signifies an Intelligence Quotient score of less than 70. Intellectual disability is generally defined by an IQ score below 70, along with significant limitations in adaptive behaviors that affect everyday general living. The cutoff of 70 is somewhat arbitrary but serves as a practical threshold for identifying individuals who may require support.

## **Why Each Wrong Option is Incorrect**
* **Option A:** An IQ score of 85 is well within the normal range, which typically spans from 85 to 115. Therefore, it does not indicate mental retardation or intellectual disability.
* **Option B:** An IQ score of 50 is indeed below the threshold for intellectual disability but specifying "less than" 70 as the diagnostic criterion makes the most accurate and encompassing choice.
* **Option D:** An IQ score of 40 is significantly below the threshold for intellectual disability, but like option B, it doesn't represent the diagnostic cutoff.

## **Clinical Pearl / High-Yield Fact**
It's crucial to remember that the diagnosis of intellectual disability involves more than just a low IQ score; it also requires significant limitations in adaptive behaviors. The assessment of intellectual disability should be multidimensional, considering both intellectual and adaptive functioning.

## **Correct Answer:** . 70