Characteristic physical features: - Upslanting palpebral fissures; epicanthal folds; Brushfield spots (white speckling on iris) - Flat facial profile; depressed nasal bridge; small ears - Small mouth; protruding tongue (macroglossia relative to mouth size) - Short neck; excess skin at nape of neck - Single palmar crease (simian crease) — also seen in normal children - Hypotonia (low muscle tone) — prominent early finding - Short stature; brachycephaly (flat occiput) - Short, broad hands; clinodactyly (curved 5th finger) - Wide gap between 1st and 2nd toes Associated clinical features: - Congenital heart disease: occurs in roughly half of infants; an atrioventricular septal defect is particularly characteristic and may be clinically silent early - Developmental delay and intellectual disability: degree varies, and skills continue to develop at an individual pace - Hypothyroidism: common and potentially subtle because fatigue, constipation, hypotonia, dry skin, and cognitive change may resemble baseline features; laboratory testing is more reliable than symptoms alone - Atlantoaxial instability: new neck pain, persistent head tilt, gait change, hand clumsiness, hyperreflexia, or loss of motor skills suggests cord involvement - Hearing...