A 56-year-old woman presents to the emergency department with a 3-day history of gradually worsening pain and swelling in her left knee.  She denies injury, fever, or other complaints. She has never experienced this type of pain before, and she states that she is fairly active.  

The patient’s vital signs are normal; physical examination is normal except for a small left knee effusion and slightly limited range of motion. The differential diagnosis includes a torn meniscus, osteoarthritis, and infection. Laboratory testing reveals a normal complete blood cell count. Radiographic examination of the left knee is performed.

What does the image show? What is the next best step in treatment?

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