A 59-year-old patient comes to the local ER with a swollen, tender knee that started yesterday. He returned home two days ago from a five-day hike on the Appalachian Trail. He reports no recent or previous injury to the knee and any history of previous inflammatory joint disease. Vital signs: temperature is 36.5 °C (97.7 °F), pulse is 80 beats/minute, blood pressure is 139/75 mmHg, and respiratory rate is 22 breaths/minute. His lower extremities are marked with abrasions in various stages of healing. He holds the knee in full extension. The knee is swollen, reddened, and tender, and it feels warm. Which of the following is the most appropriate next step? A. Knee aspiration B. Magnetic resonance imaging (MRI) of the knee C. Prescription for colchicine D. Prescription for full-dose acetaminophen E. X-ray of the knee, including sunrise and standing films