Final answer:
The nurse practitioner would know the treatment for Osgood-Schlatter Disease has been effective when the child no longer complains of pain in the lower knee during exercise.
Step-by-step explanation:
When caring for a child with Osgood-Schlatter Disease, the nurse practitioner would know her treatment has been effective when the child no longer complains of pain at the lower knee during exercise. Osgood-Schlatter Disease is a condition that causes pain and swelling below the kneecap, especially during physical activity. If the child no longer experiences pain in this area while exercising, it indicates that the treatment has been successful in managing the symptoms.