Sonic Hedgehog (SHH) is secreted from the zone of polarizing activity (ZPA) of the limb bud. Loss-of-function mutations in SHH cause which specific limb anomaly?

• A Polydactyly (preaxial)
• B Oligodactyly affecting postaxial digits ✓
• C Adactyly with absent radius (preaxial deficiency)
• D Syndactyly of all digits

Explanation

SHH from the ZPA specifies posterior (postaxial/ulnar) digit identity. Loss of SHH signalling leads to loss of posterior digit specification, producing oligodactyly or absence of ulnar/fibular digits. Gain-of-function SHH ectopically activates preaxial polydactyly (extra thumb). Adactyly with absent radius is a VACTERL-associated anomaly not primarily caused by SHH. Syndactyly results from failure of apoptosis between interdigital webbing (apoptosis is BMP-driven).

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.