In achondroplasia, which limb segments are disproportionately shortened?

• A. Distal segments
• B. Proximal segments
• C. Both distal and proximal equally
• D. Spinal segments

Answer: (B)

Achondroplasia shows a rhizomelic pattern of shortening, where the proximal limb segments—the humerus and femur—are disproportionately shortened. This comes from a FGFR3 mutation that disrupts chondrocyte proliferation in growth plates, most strongly affecting endochondral ossification of the proximal long bones. As a result, the upper arms and thighs are much shorter relative to the forearms and lower legs, which are the distal segments and are comparatively less affected. Spinal segments aren’t the limb segments in question, so they don’t define this pattern.