Final answer:
The nurse practitioner may suspect a meniscus injury, as the symptoms of knee "locking" and instability after running are characteristic of this type of injury.
Step-by-step explanation:
Based on the subjective findings that the individual experiences knee "locking up" and a feeling of instability or being "wobbly" after a long run, the nurse practitioner may suspect a meniscus injury. These symptoms are indicative of issues which can occur within the knee joint, especially in sports-related activities that involve a lot of knee movement and pressure on the joint. The detailed description of the symptoms, combined with the knowledge that the patient is a runner, points towards a possible meniscus injury rather than an ACL, MCL, or bursitis, though a clinical examination would be necessary to confirm the diagnosis.