A 7-year-old boy presents with a supracondylar fracture of the humerus (Gartland Type II) sustained during a fall from a tree. On examination, the radial pulse is palpable but weak, and there is mild swelling over the anterior elbow. A 9-year-old girl with a similar mechanism of injury has a supracondylar fracture (Gartland Type III) with absent radial pulse and severe anterior swelling. Which clinical finding best distinguishes a Type III supracondylar fracture from Type II?