**Core Concept**
Profound mental retardation (now termed intellectual disability) is defined by significant limitations in intellectual functioning and adaptive skills. The severity of intellectual disability is classified based on IQ scores, with profound disability corresponding to the most severe level of cognitive impairment.

**Why the Correct Answer is Right**
In the standard classification, profound intellectual disability is defined as an IQ score of **less than 20**. This level reflects severe cognitive deficits, with minimal ability to learn, reason, or perform daily living tasks independently. It is associated with profound limitations in communication, self-care, and social interaction, and often requires lifelong support. The diagnosis is based on standardized IQ testing and adaptive functioning.

**Why Each Wrong Option is Incorrect**
Option A: A score of 50–69 corresponds to mild intellectual disability, not profound.
Option B: A score of 35–49 represents moderate intellectual disability, which is less severe than profound.
Option C: A score of 20–34 falls under borderline or mild intellectual disability, still not classified as profound.

**Clinical Pearl / High-Yield Fact**
Remember: **IQ < 20 = Profound intellectual disability**. This threshold is critical in differential diagnosis and planning for long-term care. Always correlate IQ with adaptive functioning for accurate assessment.

✓ Correct Answer: D. < 20