A 35-year-old woman with a fibula fracture exhibits weakness in which specific leg motion due to common peroneal nerve injury?

Weakness in dorsiflexion occurs due to common peroneal nerve injury since this nerve is responsible for innervating the muscles in the anterior compartment of the leg. These muscles, particularly the tibialis anterior, play a key role in lifting the foot upward (dorsiflexion).

In cases of common peroneal nerve injury, the patient may present with difficulty lifting the front part of the foot, which can result in a characteristic "foot drop" gait, where the toe drags during walking because the individual cannot dorsiflex the foot. This motion is crucial for clear foot clearance during ambulation.

The other options relate to movements that are innervated by different nerves. Plantar flexion is primarily controlled by the tibial nerve, flexion at the knee involves the sciatic nerve primarily, and hip abduction utilizes the superior gluteal nerve. Therefore, the specific weakness seen in dorsiflexion clearly points to the involvement of the common peroneal nerve.