A 5-year-old child presents with a 2-day history of the atraumatic onset of pain, erythema, and swelling of the right knee joint. The child is febrile with an elevated white blood cell count. The differential diagnosis includes:

A. Acute rheumatic fever.

B. Leukemia.

C. Scurvy.

D. Acute septic arthritis.

E. Acute juvenile rheumatoid arthritis.

DISCUSSION: All of the above conditions may present with the clinical and laboratory findings described above. Aspiration of the joint and examination of the synovial fluid is required to rule out pyogenic arthritis. Ancillary laboratory testing for juvenile rheumatoid arthritis and acute rheumatic fever may be required.