A 6-year-old girl presents to the orthopedic clinic 3 weeks after a supracondylar fracture of the humerus that was treated with closed reduction and percutaneous pinning. She now complains of inability to extend her right thumb and weakness in gripping objects. Examination shows loss of thumb interphalangeal (IP) joint extension and weakness of finger flexion in the index and middle fingers. The radial nerve is intact. Which nerve is most likely injured?