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Down’s, Edwards’, Patau’s Syndromes | MRCP Part 1

TL;DR

Genetics: Down’s, Edwards’, Patau’s Syndromes are core MRCP Part 1 topics requiring rapid recognition of phenotype, karyotype, and screening patterns. Down’s (Trisomy 21) presents with hypotonia and AVSD, Edwards’ (Trisomy 18) with clenched fists, and Patau’s (Trisomy 13) with midline defects. Questions commonly test antenatal screening results and distinguishing features.


Why this matters

Chromosomal trisomies are among the most tested genetics topics in MRCP Part 1, often appearing as short clinical stems requiring immediate pattern recognition. The exam expects you to differentiate between syndromes using one or two key clues, rather than recalling exhaustive lists.

This topic also integrates antenatal screening, paediatrics, and molecular genetics—making it highly examinable. Begin with the MRCP Part 1 overview and reinforce learning with Free MRCP MCQs.


Core sections

1. The Big Three Trisomies (High-Yield Table)

Syndrome

Chromosome

Hallmark Features

Prognosis

Down’s

21

Hypotonia, AVSD, single palmar crease

Good survival

Edwards’

18

Clenched fists, rocker-bottom feet

Poor (<1 year)

Patau’s

13

Midline defects, polydactyly

Very poor

👉 Memorising this table can solve multiple MRCP questions instantly.

2. Down’s Syndrome (Trisomy 21)

Genetics

  • 95% due to meiotic non-disjunction

  • Others: Robertsonian translocation, mosaicism

Clinical Features

  • Neonatal hypotonia (key clue)

  • Flat facial profile

  • Epicanthic folds

  • Single palmar crease

  • Brushfield spots

System Associations

  • Cardiac: Atrioventricular septal defect (AVSD)

  • GI: Duodenal atresia

  • Haematology: Acute leukaemia risk

  • Endocrine: Hypothyroidism

Exam Insight

A neonate with hypotonia + duodenal atresia = think Down’s syndrome

3. Edwards’ Syndrome (Trisomy 18)

Clinical Features

  • Prominent occiput

  • Micrognathia

  • Low-set ears

  • Clenched fists with overlapping fingers

  • Rocker-bottom feet

System Associations

  • Congenital heart disease (VSD common)

  • Renal abnormalities

Prognosis

  • Severe developmental delay

  • High mortality in first year

Exam Insight

“Clenched fists” is the strongest clue for Edwards’ in MRCP

4. Patau’s Syndrome (Trisomy 13)

Clinical Features

  • Holoprosencephaly

  • Cleft lip/palate

  • Polydactyly

  • Microphthalmia

System Associations

  • Cardiac defects

  • Renal anomalies

Prognosis

  • Very poor survival (weeks to months)

Exam Insight

Midline defects = Patau’s until proven otherwise

5. Antenatal Screening Patterns (Very High Yield)

First Trimester Screening

  • Nuchal translucency

  • β-hCG

  • PAPP-A

Patterns

  • Down’s: ↑ β-hCG, ↓ PAPP-A

  • Edwards’/Patau’s: ↓ β-hCG, ↓ PAPP-A

Second Trimester (Quad Test)

  • AFP, hCG, Estriol, Inhibin A

Down’s Pattern

  • ↓ AFP

  • ↓ Estriol

  • ↑ hCG

  • ↑ Inhibin A

6. Mechanisms of Trisomy

  • Meiotic non-disjunction (most common)

  • Robertsonian translocation

  • Mosaicism

Exam Trap

Translocation Down’s has higher recurrence risk—important in counselling questions

7. Rapid Comparison (Exam Recall)

  1. Hypotonia → Down’s

  2. Clenched fists → Edwards’

  3. Midline defects → Patau’s

  4. Best prognosis → Down’s

  5. Worst prognosis → Patau’s

8. 10 High-Yield Points

  1. Down’s = most common viable trisomy

  2. Edwards’ = clenched hands hallmark

  3. Patau’s = midline defects dominate

  4. Maternal age increases risk

  5. Down’s linked to AVSD

  6. Edwards’ = severe developmental delay

  7. Patau’s = holoprosencephaly

  8. Screening markers differ significantly

  9. Non-disjunction is most common cause

  10. Translocation affects recurrence risk


Practical examples / mini-cases

MCQ

A newborn has low birth weight, micrognathia, clenched fists, and overlapping fingers. What is the diagnosis?

A. Down’s syndromeB. Edwards’ syndromeC. Patau’s syndromeD. Turner syndrome

Answer: B. Edwards’ syndrome

Explanation: The presence of clenched fists with overlapping fingers is a classic feature of Edwards’ syndrome. Down’s typically presents with hypotonia, while Patau’s involves midline defects.


Doctor revising for exam

Common pitfalls (5 bullets)

  • Confusing screening results between trisomies

  • Missing “clenched fists” clue

  • Assuming similar survival across syndromes

  • Forgetting translocation recurrence risk

  • Mixing polydactyly (Patau’s) with Down’s features


FAQs

1. Which trisomy is most commonly tested in MRCP Part 1?

Down’s syndrome is most frequently tested due to its prevalence and wide range of clinical associations.

2. What is the quickest way to identify Edwards’ syndrome?

Look for clenched fists with overlapping fingers—this is the most distinctive feature.

3. What are the key features of Patau’s syndrome?

Midline defects such as cleft palate, holoprosencephaly, and polydactyly are typical.

4. What is the most common cause of trisomies?

Meiotic non-disjunction is responsible for the majority of cases.

5. Why is screening important in MRCP questions?

Screening patterns (β-hCG, PAPP-A, AFP) are frequently tested in data interpretation questions.


Ready to start?

Strengthen your retention with:

For a complete roadmap, revisit the MRCP Part 1 overview.


Sources

 
 
 

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