Chromosomal Disorders & SyndromesMarch 21, 20265 min read

Q-Bank Breakdown: Down syndrome (Trisomy 21) — Why Every Answer Choice Matters

Clinical vignette on Down syndrome (Trisomy 21). Explain correct answer, then systematically address each distractor. Tag: Genetics > Chromosomal Disorders & Syndromes.

Q-Bank Breakdown: Down syndrome (Trisomy 21) — Why Every Answer Choice Matters

Tag: Genetics > Chromosomal Disorders & Syndromes

Down syndrome (Trisomy 21) is a Step staple because it blends genetics (nondisjunction vs translocation) with multi-system clinical findings and high-yield screening/diagnostic testing. In this breakdown, we’ll work through a classic vignette and then systematically dismantle every distractor—the skill that separates “I knew it” from “I can’t miss it.”

Clinical Vignette (Q-bank style)

A 37-year-old woman delivers a newborn with upslanting palpebral fissures, a single transverse palmar crease, hypotonia, and a systolic murmur. Echocardiogram shows an endocardial cushion defect. The mother asks about recurrence risk and genetic mechanisms. Karyotype of the infant shows 47,XY,+21 in all cells.

Question: What is the most likely underlying mechanism?

✅ Correct Answer: Meiotic nondisjunction (most often maternal meiosis I)

Why the Correct Answer Is Correct (Down syndrome essentials)

Core concept

  • Trisomy 21 most commonly results from meiotic nondisjunction, typically in maternal meiosis I.
  • Risk increases strongly with advanced maternal age due to age-related meiotic spindle/segregation issues.

Expected findings in Down syndrome (high yield)

Physical / neuro

  • Hypotonia
  • Upward-slanting palpebral fissures
  • Epicanthal folds
  • Flat facial profile
  • Single transverse palmar crease
  • “Sandal gap” (increased space between 1st and 2nd toes)
  • Intellectual disability (variable)

Cardiac (Step favorites)

  • Endocardial cushion defects / AV septal defect
  • VSD/ASD can also occur

GI

  • Duodenal atresia (double bubble, vomiting)
  • Hirschsprung disease (less common but testable)

Heme/onc

  • Increased risk of acute lymphoblastic leukemia (ALL) and acute megakaryoblastic leukemia (AML M7)
  • Transient myeloproliferative disorder in neonates

Neurodegeneration

  • Early-onset Alzheimer disease (APP gene on chromosome 21)

Endocrine

  • Hypothyroidism

The Key Twist: Mechanism Matters for Recurrence Risk

This vignette explicitly gives:

  • 37-year-old mother (age risk)
  • Karyotype: 47,XY,+21 in all cells (full trisomy)
  • No mention of translocation pattern (e.g., 46,XX,rob(14;21),+21)

That combination points to meiotic nondisjunction, which has a recurrence risk that is lower than translocation-mediated cases (though still above baseline, and maternal age remains relevant).

Distractor Breakdown: Why Every Wrong Answer Is Wrong (and when it’s right)

Below are classic answer choices that show up alongside Down syndrome. Knowing their signatures prevents “pattern-matching traps.”

Distractor 1: Robertsonian translocation (e.g., t(14;21) or t(21;21))

Why it’s tempting

Robertsonian translocation is a well-known cause of Down syndrome and is often tested for recurrence risk and family counseling.

Why it’s wrong in this vignette

  • The karyotype given is 47,XY,+21 (simple trisomy), not a translocation pattern.
  • Translocation Down often shows 46 chromosomes with extra 21 material attached to another chromosome (classically chromosome 14).

When this distractor becomes correct

Suspect translocation Down when:

  • Down syndrome in a young mother (no maternal age risk)
  • Recurrent Down syndrome in a family
  • Karyotype suggests translocation:
    • Example: 46,XX,rob(14;21),+21
  • Important recurrence counseling:
    • If a parent is a balanced carrier, recurrence risk can be significant
    • t(21;21) carriers can have very high recurrence risk (conceptually near 100% for viable offspring having Down)

USMLE tip:
If the stem emphasizes multiple affected siblings or a parent with a balanced translocation, think Robertsonian.

Distractor 2: Mosaic Down syndrome (postzygotic mitotic nondisjunction)

Why it’s tempting

“Mosaicism” shows up as a mechanism for genetic variability and sometimes milder phenotypes.

Why it’s wrong in this vignette

  • The karyotype is 47,XY,+21 in all cells—that’s not mosaic.
  • Mosaic Down would show two cell lines, e.g.:
    • 46,XY and 47,XY,+21

When this distractor becomes correct

Pick mosaicism when:

  • Karyotype shows mixed cell populations
  • Phenotype is milder or atypical
  • Mechanism: mitotic nondisjunction after fertilization

Distractor 3: Trisomy 18 (Edwards syndrome)

Why it’s tempting

Another autosomal trisomy with severe congenital anomalies and growth restriction.

Why it’s wrong here

Edwards syndrome hallmark findings differ:

  • Clenched fists with overlapping fingers
  • Rocker-bottom feet
  • Prominent occiput
  • Micrognathia
  • Severe intellectual disability, often death in the first year

The vignette’s signs (single palmar crease, upslanting fissures, endocardial cushion defect) point to Down, not Edwards.

Distractor 4: Trisomy 13 (Patau syndrome)

Why it’s tempting

A trisomy with multi-system anomalies and high mortality.

Why it’s wrong here

Patau classic triad:

  • Cleft lip/palate
  • Holoprosencephaly
  • Polydactyly Plus:
  • Microphthalmia
  • Cutis aplasia (scalp defects)
  • Severe congenital heart disease

Not the phenotype described.

Distractor 5: Turner syndrome (45,X)

Why it’s tempting

Also commonly tested with congenital heart disease and characteristic physical findings.

Why it’s wrong here

Turner affects phenotypic females and features:

  • Webbed neck
  • Shield chest, widely spaced nipples
  • Streak ovaries → primary amenorrhea, infertility
  • Coarctation of the aorta (classically)
  • Bicuspid aortic valve

This vignette is a male newborn with Down features and AV canal defect.

Distractor 6: Klinefelter syndrome (47,XXY)

Why it’s tempting

A common aneuploidy with a numeric karyotype starting with 47.

Why it’s wrong here

Klinefelter presents later (often at puberty/adulthood) with:

  • Tall stature
  • Small, firm testes
  • Infertility
  • Gynecomastia
  • ↓ testosterone, ↑ LH/FSH

It does not explain classic Down facial/palmar features or AV septal defect.

Distractor 7: Cri-du-chat syndrome (5p deletion)

Why it’s tempting

A classic chromosomal disorder with recognizable clinical clues.

Why it’s wrong here

Cri-du-chat hallmark:

  • High-pitched “cat-like” cry
  • Microcephaly
  • Intellectual disability
  • Characteristic facial features (not Down’s pattern)

Mechanism is deletion, not trisomy.

Distractor 8: 22q11.2 deletion syndrome (DiGeorge)

Why it’s tempting

Congenital heart disease + dysmorphic features can overlap on exams.

Why it’s wrong here

DiGeorge classic triad:

  • Cleft palate
  • Hypocalcemia (↓ PTH) → tetany, seizures
  • Thymic aplasia → T-cell deficiency, recurrent viral/fungal infections
    Also: conotruncal cardiac defects (e.g., truncus arteriosus, tetralogy of Fallot)

Down syndrome is more tied to endocardial cushion defects and characteristic facies/palmar crease.

Testing & Screening: Step-Ready Down Syndrome Workflow

Prenatal screening (non-diagnostic)

Second trimester quad screen (15–20 weeks):

  • ↓ AFP
  • ↑ β-hCG
  • ↓ estriol
  • ↑ inhibin A

(“Down: hCG and inhibin up; AFP and estriol down.”)

First trimester: increased nuchal translucency.

Diagnostic testing (definitive)

  • Chorionic villus sampling (CVS): ~10–13 weeks
  • Amniocentesis: ≥15 weeks
  • Karyotype/FISH/microarray used depending on the question

High-Yield Takeaways (Rapid Review)

  • Most Down syndrome = maternal meiosis I nondisjunction → 47,+21
  • Advanced maternal age increases nondisjunction risk (not usually translocation risk)
  • AV septal defect/endocardial cushion defect is the classic congenital heart lesion
  • Duodenal atresia + “double bubble” is a classic GI association
  • Quad screen: ↓AFP, ↑hCG, ↓estriol, ↑inhibin A
  • If Down occurs in a young mother or recurs in siblings, think Robertsonian translocation
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