Educational Disclaimer: This case study is for educational and informational purposes only. It does not constitute medical advice, clinical guidance, or a substitute for professional psychiatric evaluation. All diagnostic criteria referenced are from the DSM-5-TR (APA, 2022). Clinicians should rely on their professional training, direct patient assessment, and current evidence-based guidelines when making diagnostic and treatment decisions.

Clinical Vignette

Patient: "Marcus," 14-year-old 8th grader in a special education program, referred for comprehensive evaluation as part of his transition planning.

Chief Concern: Special education teacher: "Marcus is a kind, motivated student who struggles significantly with academic material across all subjects. He reads at a 3rd-grade level, does math at a 2nd-grade level, and needs support for daily tasks that his peers handle independently. We need a comprehensive evaluation for his transition to high school."

History of Present Illness: Marcus has been in special education since kindergarten after developmental testing revealed global delays. He met motor milestones on time but was late for language milestones (first words at 24 months, sentences at 42 months). Cognitive testing: Full-Scale IQ = 62 (WISC-V). Adaptive Behavior Assessment (Vineland-3): Communication: below age expectations (can hold simple conversations but struggles with complex topics, does not understand abstract concepts or figurative language). Daily Living Skills: below age expectations (can dress independently, performs basic hygiene, but cannot manage money, cook without supervision, or navigate public transportation alone). Socialization: below age expectations (has friends, understands basic social rules, but misses social nuances, is easily manipulated by peers, and cannot recognize deceptive intentions). Academic functioning: reading 3rd-grade level, math 2nd-grade level, writing limited to short simple sentences. He is aware of his differences: 'I know I'm not as fast as the other kids, but I try really hard.'

Medical History: Premature birth (34 weeks). No seizures. No genetic syndromes identified. Normal hearing and vision.

Mental Status Exam: Cooperative, eager to please. Eye contact appropriate. Speech simple, grammatically correct for short sentences. Vocabulary limited. Could not explain abstract concepts (e.g., 'What does justice mean?' — 'I don't know that word'). Affect pleasant, congruent. Followed simple 2-step instructions. Struggled with 3-step instructions. No psychotic symptoms. No behavioral problems. Insight: partial awareness of his differences.

Step 1: Intellectual Disability DSM-5-TR Criteria

Criterion A: Deficits in intellectual functions confirmed by both clinical assessment and individualized, standardized intelligence testing.

WISC-V Full-Scale IQ = 62 (approximately 2 standard deviations below mean). Clinical presentation consistent: limited abstract reasoning, simplified language, academic achievement far below grade level. MET.

Criterion B: Deficits in adaptive functioning that result in failure to meet developmental and sociocultural standards for personal independence and social responsibility, in ≥1 domain (conceptual, social, practical).

Conceptual: reads at 3rd-grade level, math at 2nd-grade level, cannot understand abstract concepts. Social: misses nuances, easily manipulated, cannot recognize deceptive intentions. Practical: cannot manage money, cook independently, or navigate transportation. All 3 domains impaired. MET — ALL 3 DOMAINS.

Criterion C: Onset of intellectual and adaptive deficits during the developmental period.

Developmental delays noted from infancy (language milestones delayed). Special education since kindergarten. MET.

Step 2: Severity Classification

Domain Mild ID (IQ ~50-70) This Patient
Conceptual Can learn academic skills to approximately 6th-grade level. Difficulty with abstract thinking, executive function. Mild: reading 3rd-grade (age 14), progressing slowly
Social Social judgment less mature. Communication more concrete. Risk of social manipulation. Mild: basic social skills intact, nuances missed, easily manipulated
Practical Can independently manage personal care. Needs support for complex tasks (finances, transportation, grocery shopping). Mild: personal care independent, complex daily living requires support
Overall severity Can often live independently with support. Can work in competitive employment with accommodations. Severity: MILD

DSM-5-TR Emphasis on Adaptive Functioning

DSM-5-TR emphasizes that severity classification is based on ADAPTIVE FUNCTIONING rather than IQ score alone. IQ testing has measurement error (standard error ± 5 points) and does not capture real-world functioning. Marcus's adaptive functioning in all 3 domains, consistent with mild severity, determines his classification.

Diagnostic Formulation

Diagnostic Conclusion

Intellectual Disability (Intellectual Developmental Disorder), Mild Severity (F70): All 3 DSM-5-TR criteria met. WISC-V IQ = 62. Adaptive deficits in all 3 domains (conceptual, social, practical). Onset during developmental period. Severity: mild (based on adaptive functioning). Transition planning recommendations: (1) Continued modified academic curriculum targeting functional literacy and numeracy. (2) Social skills training emphasizing recognition of manipulation/deception. (3) Independent living skills training (budgeting, cooking, transportation). (4) Pre-vocational training aligned with strengths and interests. (5) Explore supported employment options.

Teaching Points

  • DSM-5-TR names this condition 'Intellectual Disability (Intellectual Developmental Disorder).' The parenthetical acknowledges WHO terminology. The DSM-5-TR specifically avoids the older term 'mental retardation,' which is no longer used in clinical or legal contexts.
  • DSM-5-TR classifies severity (mild, moderate, severe, profound) based on ADAPTIVE FUNCTIONING across 3 domains (conceptual, social, practical), NOT IQ score alone. This represents a shift from DSM-IV, which used IQ ranges as the primary severity classifier. The rationale: adaptive functioning better predicts real-world outcomes and support needs than IQ alone.
  • IQ testing has a standard error of measurement of approximately ±5 points. Marcus's IQ of 62 represents a range of approximately 57-67. This measurement error is why DSM-5-TR de-emphasizes rigid IQ cutoffs for severity classification and prioritizes adaptive behavior assessment.
  • The distinction between Intellectual Disability and Specific Learning Disorder: ID involves GLOBAL cognitive deficit (all domains affected). SLD involves SPECIFIC deficit (one or two academic areas) with normal overall cognitive ability. Marcus's global deficits across reading, math, abstract reasoning, AND adaptive functioning distinguish ID from SLD.
  • Vulnerability to social manipulation is a clinically significant consequence of mild ID. Marcus's difficulty recognizing deceptive intentions places him at risk for exploitation. Social skills training should specifically address recognizing manipulation, deception, and abuse.