You’ve probably seen a question where the stem screams “imprinting,” but the answer choices all look like slightly different flavors of developmental delay. This is exactly where USMLE loves to live: Prader-Willi vs Angelman. The trick isn’t just knowing the syndromes—it’s knowing how to rule out the distractors fast based on who’s missing which allele, in which parent, at which locus, and what the phenotype looks like.
Tag: Genetics > Chromosomal Disorders & Syndromes
The Clinical Vignette (Classic Q-Bank Style)
A 6-year-old boy is brought to clinic for behavioral issues. His parents report hyperphagia and rapid weight gain since age 2. He has mild intellectual disability, short stature, small hands and feet, and a history of neonatal hypotonia with poor feeding requiring special nipples. Physical exam shows hypogonadism and obesity. Genetic testing suggests a disorder due to loss of expression of paternally inherited genes on chromosome 15.
What is the most likely diagnosis?
The Correct Answer: Prader-Willi Syndrome (PWS)
Why it’s PWS
This stem is a greatest-hits album:
- Neonatal hypotonia + poor feeding → early PWS clue
- Later hyperphagia → obesity (often severe)
- Hypogonadism (cryptorchidism, delayed puberty)
- Short stature, small hands/feet
- Mild-to-moderate intellectual disability and behavioral issues
The genetics (the part Step loves)
Prader-Willi = loss of paternally expressed genes at 15q11–q13.
Most common mechanisms:
- Paternal deletion of 15q11–q13
- Maternal uniparental disomy (UPD) (child inherits two maternal copies of chromosome 15)
- Less commonly: imprinting center defects
High-yield memory hook:
- PWS = “Paternal” problem → hyperphagia/obesity
- Angelman = “mAternal” problem → Ataxia + seizures + inappropriate laughter
Build the Mental Model: What’s Special About 15q11–q13?
This region is imprinted, meaning gene expression depends on parent of origin.
Quick table: PWS vs Angelman
| Feature | Prader-Willi | Angelman |
|---|---|---|
| Missing gene expression | Paternal (15q11–q13) | Maternal (15q11–q13) |
| Typical mechanism | Paternal deletion or maternal UPD | Maternal deletion or paternal UPD |
| Key clinical clues | Hypotonia → hyperphagia, obesity, hypogonadism, small hands/feet | Severe ID, ataxia, seizures, inappropriate laughter (“happy puppet”) |
| Feeding pattern | Poor feeding as infant → hyperphagia later | Not classic for hyperphagia |
| Buzzwords | “Floppy infant,” “insatiable appetite” | “Happy,” “ataxic,” “seizures” |
Now the USMLE Skill: Why Every Distractor Is Wrong (and How to Kill It Quickly)
Below are common answer choices that appear alongside PWS/Angelman. Your job on test day: match one or two discriminating clinical features and a signature genetic mechanism.
Distractor 1: Angelman Syndrome (The “same locus” trap)
Why they want you to pick it
Angelman also involves 15q11–q13 imprinting, so students panic and choose it whenever they see chromosome 15.
Why it’s wrong here
Angelman is characterized by:
- Severe intellectual disability
- Seizures
- Ataxia
- Inappropriate laughter and frequent smiling (“happy puppet”)
- Often microcephaly
This vignette instead gives:
- Hyperphagia/obesity
- Hypotonia in infancy
- Hypogonadism
- Small hands/feet
Those are Prader-Willi, not Angelman.
High-yield gene note: Angelman often involves loss of maternal UBE3A expression (especially in neurons).
Distractor 2: Fragile X Syndrome
Why it looks tempting
Fragile X is a common cause of intellectual disability and may show behavioral issues.
How to rule it out fast
Fragile X key clues:
- Long face, large ears
- Macroorchidism (post-pubertal)
- Autism/ADHD features
- Mitral valve prolapse
- Genetics: CGG trinucleotide repeat expansion in FMR1 (X chromosome) → anticipation, methylation, decreased FMRP
This stem has hypogonadism and small hands/feet, not macroorchidism. Also hyperphagia is not a Fragile X feature.
USMLE one-liner: Fragile X = most common inherited cause of intellectual disability; PWS/Angelman = imprinting disorders.
Distractor 3: Down Syndrome (Trisomy 21)
Why it’s on the list
Down is the most common chromosomal aneuploidy students recognize, and Q-banks love to see if you’re overusing it.
How to rule it out fast
Down syndrome clues:
- Upslanting palpebral fissures, epicanthal folds
- Single transverse palmar crease
- Duodenal atresia (“double bubble”), Hirschsprung disease
- Endocardial cushion defects
- Brushfield spots
- Increased risk: ALL, AML (M7), early-onset Alzheimer
None of those are in the stem. Plus, Down is an aneuploidy, not an imprinting disorder involving 15q11–q13.
Distractor 4: Turner Syndrome (45,X)
Why it’s tempting
Short stature and hypogonadism show up in the stem.
How to rule it out fast
Turner features:
- Phenotypic female with streak ovaries
- Webbed neck, shield chest
- Coarctation of the aorta (classically) / bicuspid aortic valve
- Primary amenorrhea, infertility
- Cystic hygroma in infancy
This patient is a boy, and the classic PWS combo (hyperphagia/obesity + small hands/feet + neonatal hypotonia) doesn’t fit Turner.
Distractor 5: Klinefelter Syndrome (47,XXY)
Why it’s tempting
Hypogonadism in a male = students jump to Klinefelter.
How to rule it out fast
Klinefelter clues:
- Tall stature with long limbs
- Small, firm testes
- Gynecomastia
- Infertility (azoospermia)
- Learning difficulties, but not typically the PWS feeding trajectory
- Genetics: extra X chromosome; Barr body present
PWS usually has short stature and a distinct hyperphagia/obesity phenotype with early hypotonia.
Distractor 6: Beckwith-Wiedemann Syndrome (Imprinting, but different locus)
Why it’s on the list
It’s another classic imprinting disorder—USMLE often pairs imprinting syndromes together.
How to rule it out fast
Beckwith-Wiedemann features:
- Macroglossia
- Omphalocele/umbilical hernia
- Organomegaly
- Hemihyperplasia
- Neonatal hypoglycemia
- Increased risk of Wilms tumor and hepatoblastoma
- Imprinting dysregulation at 11p15 (IGF2/H19 region)
This stem is obesity/hyperphagia/hypotonia—not overgrowth and tumor risk.
Distractor 7: DiGeorge Syndrome (22q11 deletion)
Why it’s tempting
Developmental issues can overlap broadly, and q-banks like to see if you can separate syndromic patterns.
How to rule it out fast
DiGeorge = CATCH-22:
- Cardiac defects (conotruncal: truncus arteriosus, tetralogy of Fallot)
- Abnormal facies
- Thymic aplasia → T-cell deficiency
- Cleft palate
- Hypocalcemia (↓PTH)
Not a match for hyperphagia/obesity/hypogonadism.
High-Yield USMLE Imprinting Pearls (Memorize These)
1) Same chromosomal region, opposite parent
- Prader-Willi: loss of paternal expression at 15q11–q13
- Angelman: loss of maternal expression at 15q11–q13
2) Mechanisms you should be able to name
- Deletion
- Uniparental disomy (UPD)
- PWS commonly maternal UPD 15
- Angelman can be paternal UPD 15
- Imprinting defects (imprinting center abnormalities)
3) Clinical “two-phase” feeding pattern is a PWS giveaway
- Infancy: hypotonia + poor feeding
- Childhood: hyperphagia → obesity
Rapid-Fire Step-Style Differentiation (If You Only Have 10 Seconds)
- Obesity + hyperphagia + small hands/feet + hypotonia → Prader-Willi
- Seizures + ataxia + inappropriate laughter → Angelman
- Macroglossia + omphalocele + hypoglycemia → Beckwith-Wiedemann
- CATCH-22 → DiGeorge
- Long face + macroorchidism → Fragile X
- Endocardial cushion + duodenal atresia → Down
Takeaway: How to “Win” These Questions
When the vignette hints at imprinting, don’t stop at “chromosome 15.” Lock onto:
- The phenotype anchor (hyperphagia/obesity vs ataxia/laughter/seizures)
- Parent-of-origin language (paternal vs maternal)
- Mechanism (deletion vs UPD)
That’s how you turn a tricky two-syndrome question into a fast, confident pick—and avoid getting baited by look-alike genetic distractors.