A 35-year-old woman is brought to the emergency department because of worsening pain and swelling of her right knee
for the past 2 days. She has been taking acetaminophen for the knee pain during the past 2 days, but the pain is worse
today. She has not had any trauma to the knee or any previous problems with her joints. She is otherwise healthy and
she currently takes an oral contraceptive. She is sexually active and has a 10-year-old son who lives with her. She is a
receptionist at a local hotel and she tells you she must stand often while working. She is 160 cm (5 ft 3 in) tall and weighs
52 kg (115 lb); BMI is 20 kg/m2
. Temperature is 37.9°C (98.9°F). The right knee is erythematous, swollen, and tender;
there is pain on movement. No other joints are affected. X-ray of the knee shows an effusion but no structural
abnormalities of the joint.
Which of the following is the most appropriate next step in diagnosis?
(A) Arthrocentesis of the knee
(

Blood cultures
(C) Complete blood count
(D) MRI of the knee
(E) Urine cultures