What is the most common cause of nongonococcal septic arthritis?

• A. Streptococcus pneumoniae
• B. Staphylococcus aureus
• C. Escherichia coli
• D. Neisseria gonorrhoeae

Answer: (C)

Staphylococcus aureus is the most common cause of nongonococcal septic arthritis. It frequently seeds the joint from the bloodstream or from a localized infection on the skin and then invades the synovium, leading to rapid inflammation and joint destruction if not treated promptly. This pattern is seen in adults across various settings, especially with risk factors like diabetes, prosthetic joints, or IV drug use. Other organisms can cause nongonococcal septic arthritis—such as Gram-negative rods like E. coli in older patients with urinary sources or certain streptococci—but none are as common as Staphylococcus aureus. Neisseria gonorrhoeae, by contrast, causes gonococcal septic arthritis, a different clinical entity.