Which autosomal dominant disorder is characterized by disproportionate short stature with short limbs?

Which autosomal dominant disorder is characterized by disproportionate short stature with short limbs?

• A. Fragile X syndrome
• B. PKU
• C. Achondroplasia
• D. Tay-Sachs disease

Answer: (C)

Disproportionate short stature with short limbs points to achondroplasia, an autosomal dominant skeletal disorder caused by activating mutations in FGFR3. In heterozygotes, endochondral bone growth in the long bones is impaired, so the proximal limbs are noticeably shortened (rhizomelia) while the trunk remains relatively normal in size. Facial features often include a large head with frontal bossing and midface hypoplasia. The heterozygous form produces the classic dwarf phenotype, and the homozygous dominant state is typically lethal in infancy, which helps explain why the condition often runs in families but isn’t uniformly severe or present in every pregnancy. Because only one mutated allele is enough to cause the condition, each child of an affected parent has a 50% chance of inheriting it. Other options represent different inheritance patterns or conditions (for example, Fragile X is X-linked; PKU and Tay-Sachs are autosomal recessive) and do not match the described presentation.

Disproportionate short stature with short limbs points to achondroplasia, an autosomal dominant skeletal disorder caused by activating mutations in FGFR3. In heterozygotes, endochondral bone growth in the long bones is impaired, so the proximal limbs are noticeably shortened (rhizomelia) while the trunk remains relatively normal in size. Facial features often include a large head with frontal bossing and midface hypoplasia. The heterozygous form produces the classic dwarf phenotype, and the homozygous dominant state is typically lethal in infancy, which helps explain why the condition often runs in families but isn’t uniformly severe or present in every pregnancy. Because only one mutated allele is enough to cause the condition, each child of an affected parent has a 50% chance of inheriting it. Other options represent different inheritance patterns or conditions (for example, Fragile X is X-linked; PKU and Tay-Sachs are autosomal recessive) and do not match the described presentation.