A 75-year-old Filipino man presented to the ambulatory medicine clinic with two days of atraumatic right knee swelling and loss of motion.
He had returned from the Philippines one week previously. He denied any constitutional symptoms such as fevers, chills, or night sweats.
His pertinent past medical history was significant only for osteoarthritis. He had become wheelchair-bound, and his examination revealed an asymmetric right knee effusion. There was obvious erythema that was warm to the touch. Crepitus was elicited bilaterally.
Passive and active range of motion in his right knee were limited, and it was tender to palpation.
Joint aspiration produced grossly milky white pus with some blood streaking due to a mildly traumatic tap. The image above shows what was seen in the aspirate.
What would you include in the differential? Which tests would you have ordered?
What's your guess at the diagnosis (and will you guess wrong initially, like the authors)?
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