A transcription factor expressed specifically in the forelimb bud mesenchyme determines forelimb versus hindlimb identity. Loss of its function in humans produces Holt-Oram syndrome with radial ray defects and septal heart defects. Which factor is this?
- A PITX1
- B TBX4
- C TBX5 ✓
- D HOXA11
Correct answer: C. TBX5
Explanation
TBX5 is the T-box transcription factor specifying forelimb identity; its mutations cause Holt-Oram syndrome, combining upper limb radial ray anomalies with atrial septal defect and conduction abnormalities. PITX1 and TBX4 specify hindlimb identity, and TBX4 mutations cause small patella syndrome. HOXA11 patterns the zeugopod (radius-ulna, tibia-fibula) level but does not confer forelimb identity or cause the cardiac component.
Reference: Larsen's Human Embryology, 5th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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