Down syndrome (trisomy 21)
What it means
Three copies of chromosome 21 (95% meiotic non-disjunction, risk ↑ with maternal age; ~4% Robertsonian translocation; ~1% mosaic). Features: hypotonia, upslanting palpebral fissures, epicanthic folds, single palmar crease, flat occiput; AVSD/VSD, duodenal atresia, hypothyroidism, leukaemia (ALL/AML), early Alzheimer's.
Worked example
Antenatal screening: combined test 11–14 wk (nuchal translucency + PAPP-A + βhCG) → higher chance offered NIPT or diagnostic CVS/amniocentesis.
In the exam
A hypotonic neonate has a single palmar crease, upslanting palpebral fissures, an atrioventricular septal defect and duodenal atresia.
What settles it
Trisomy 21 has the best survival of the autosomal trisomies; Edwards (18) and Patau (13) are usually lethal in infancy.
Classically confused with
Edwards & Patau syndromes
Source: StatPearls; NHS FASP