A 7-year-old boy is brought to the child psychiatry clinic by his parents with concerns about delayed development. He attained sitting at 12 months and walking at 24 months. He speaks in 2–3 word phrases and has difficulty understanding complex instructions. On examination, he has a broad nasal bridge, upslanting palpebral fissures, and a single palmar crease bilaterally. His IQ on formal testing is 42. The parents report he has been attending a special school for the past 2 years. What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** The combination of characteristic facial features (broad nasal bridge, upslanting palpebral fissures), single palmar crease, developmental delay, and IQ of 42 (moderate intellectual disability range: 35–49) is pathognomonic for Down syndrome (Trisomy 21). ### Diagnostic Features of Down Syndrome | Feature | Presence in This Case | Significance | |---------|----------------------|---------------| | Upslanting palpebral fissures | Yes | Classic dysmorphic feature | | Broad nasal bridge | Yes | Characteristic facies | | Single palmar crease | Yes | Present in ~45% of Down syndrome | | Developmental delay (sitting 12 mo, walking 24 mo) | Yes | Typical motor milestone pattern | | IQ 42 (moderate range) | Yes | Most common severity in Down syndrome | | Speech delay (2–3 word phrases at age 7) | Yes | Expected language profile | ### Intellectual Disability Severity Classification **High-Yield:** IQ ranges define severity: - Mild: 50–69 - Moderate: 35–49 - Severe: 20–34 - Profound: <20 This child's IQ of 42 falls squarely in the moderate range, typical for Down syndrome. ### Why Down Syndrome? **Clinical Pearl:** Down syndrome is the most common chromosomal cause of intellectual disability (1 in 700 live births). The facial dysmorphism (upslanting palpebral fissures, broad nasal bridge) combined with single palmar crease is virtually diagnostic. The developmental trajectory (sitting at 12 months, walking at 24 months) and language delay are entirely consistent with moderate intellectual disability in Down syndrome. ### Associated Features Often Present - Cardiac defects (40–50%): AV canal defect, VSD, ASD - GI anomalies: duodenal atresia, Hirschsprung disease - Hearing loss (60–70%) - Thyroid dysfunction (15–20%) - Atlantoaxial instability **Mnemonic: FACES of Down Syndrome** — Flat face, Almond-shaped eyes, Curved pinky, Ear abnormalities, Single palmar crease [cite:Kaplan & Sadock's Synopsis of Psychiatry 12e Ch 45]