Final answer:
Muscle atrophy in the gluteal region is the most likely physical finding in a patient with a superior gluteal nerve injury, which results in a waddling gait and a positive Trendelenburg sign.
Step-by-step explanation:
The most likely physical finding in a patient with an injury to the superior gluteal nerve, resulting in a waddling gait and a positive Trendelenburg sign, is muscle atrophy in the gluteal region. This nerve injury affects the gluteal muscles, especially the gluteus medius and minimus, which are key stabilizers of the pelvis during gait. The inability to properly contract these muscles leads to a downward pelvic tilt on the contralateral side when the ipsilateral foot is lifted off the ground during walking, which is noted as a positive Trendelenburg sign. Over time, disuse of the affected muscles can lead to muscle atrophy.