USMLE Step 1 · Biochemistry & Genetics

Achondroplasia vs Osteogenesis imperfecta and hypochondroplasia

How to tell Achondroplasia and Osteogenesis imperfecta and hypochondroplasia apart when a board question makes them look alike.

Fastest separator

Achondroplasia is FGFR3 short-limb dwarfism; osteogenesis imperfecta is collagen brittle-bone disease; hypochondroplasia is milder FGFR3 short stature.

Side by side

AchondroplasiaOsteogenesis imperfecta and hypochondroplasia

Key clue

Achondroplasia causes short-limb dwarfism, large head, frontal bossing, normal intelligence, and impaired endochondral bone growth from activating FGFR3 mutation.

Osteogenesis imperfecta causes fragility fractures with minimal trauma, Wormian bones, hearing loss, dentinogenesis imperfecta, and COL1A1/COL1A2 defects; hypochondroplasia is a milder FGFR3 short-stature disorder.

Mechanism

Constitutive FGFR3 signaling inhibits chondrocyte proliferation at growth plates.

COL1A1/COL1A2 defects weaken collagen in bone and connective tissue; milder FGFR3 activation causes hypochondroplasia.

Labs and tests

Rhizomelic limb shortening, frontal bossing, and normal trunk support achondroplasia.

Fragility fractures, Wormian bones, dentinogenesis imperfecta, and COL1A1/COL1A2 defects support OI; mild disproportionate short stature with fewer craniofacial clues supports hypochondroplasia.

The trap

Short stature alone is not enough; fracture history, blue sclerae, and craniofacial/growth-plate clues separate them.

The repair move

For skeletal dysplasia, separate growth plate signaling defects from collagen fragility.

Try a free question

Core

A 6-month-old infant has disproportionate short limbs (rhizomelic shortening with normal trunk length), a large head with frontal bossing, a depressed nasal bridge, lumbar lordosis, trident-shaped hand, and normal intelligence. Father has similar appearance. Genetic testing confirms an activating FGFR3 mutation. Which condition is most likely?

Show answer and explanation

Answer: A. Achondroplasia. Achondroplasia is autosomal dominant constitutive activation of FGFR3, which inhibits chondrocyte proliferation at growth plates. Most cases are de novo mutations from advanced paternal age. Features: rhizomelic limb shortening, large head with frontal bossing, lumbar lordosis, trident hand, NORMAL intelligence.

Why the other choices are tempting

B. Osteogenesis imperfecta features fragility fractures with minimal trauma, blue sclerae, dentinogenesis imperfecta, and hearing loss from COL1A1/COL1A2 collagen defect, not rhizomelic limb shortening.
C. Hypochondroplasia is a milder FGFR3 disorder with milder disproportionate short stature and fewer craniofacial features.
D. Thanatophoric dysplasia is a more severe FGFR3 mutation that is lethal in the perinatal period, with telephone-receiver femurs and severe cloverleaf skull.
E. Hurler syndrome features coarse facies, corneal clouding, and hepatosplenomegaly from alpha-L-iduronidase deficiency, not rhizomelic limb shortening alone.

Flashcards

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What clue supports Achondroplasia?

Choose Achondroplasia when the stem gives this pattern: Achondroplasia causes short-limb dwarfism, large head, frontal bossing, normal intelligence, and impaired endochondral bone growth from activating FGFR3 mutation.

What clue supports Osteogenesis imperfecta and hypochondroplasia?

Choose Osteogenesis imperfecta and hypochondroplasia when the stem gives this contrasting pattern: Osteogenesis imperfecta causes brittle bones, blue sclerae, hearing loss, dental problems, and type I collagen defects; hypochondroplasia is a milder FGFR3 short-stature disorder.

Quick answers

What is the fastest way to tell Achondroplasia from Osteogenesis imperfecta and hypochondroplasia?