Knee Pain in an 8-Year-Old Boy

Case
An 8-year-old boy presents to an urgent care center with left knee pain that he has had for 3 days. The pain is worse with walking, and his mother says that he has been limping and will not put full weight on the leg. He reports no trauma, fever, vomiting, or numbness. His mother reports that she has given him ibuprofen but that this has not relieved his pain.

View the image taken (Figure 1) and consider what your diagnosis would be.