Which disease is characterized by an overactive receptor causing long bone growth defects?

• A. Marfan Syndrome
• B. Familial Hypercholesterolemia
• C. Achondroplasia
• D. Huntington Disease

Answer: (C)

The disease characterized by an overactive receptor that leads to long bone growth defects is achondroplasia. This condition is primarily caused by a mutation in the FGFR3 gene, which encodes for a receptor that regulates bone growth. In achondroplasia, the mutation results in a gain-of-function effect on the receptor, causing it to become overactive. This overactivity inhibits the proliferation and function of chondrocytes in the growth plates of long bones, leading to insufficient endochondral ossification and therefore shorter long bones, resulting in disproportionate short stature.

The other conditions mentioned do not relate to growth defects caused by an overactive receptor specific to long bone growth. Marfan syndrome is associated with connective tissue abnormalities without the specific receptor involvement in growth. Familial hypercholesterolemia involves mutations affecting LDL receptors related to cholesterol metabolism rather than bone growth. Huntington disease is a neurodegenerative disorder caused by CAG repeat expansions, unrelated to bone development. Thus, achondroplasia is the correct answer due to its distinct mechanism directly linked to the regulation of bone growth through receptor activity.