A 27-year-old man develops acute pain and swelling of the left knee 5 days after an episode of urethritis. On physical examination, the left knee is swollen, warm, and tender to the touch. No other joints are affected. Laboratory examination of fluid aspirated from the left knee joint shows numerous neutrophils. A Gram stain of the fluid shows with gram-negative intracellular diplococci. No crystals are seen. Which of the following infectious agents is most likely responsible for his condition?

Gonorrhea should be considered the most likely cause of acute suppurative arthritis in sexually active individuals; in some cases, multiple joints can be involved. In men, urethritis may occur with a gonorrheal infection. Borrelia burgdorferi causes Lyme disease, characterized by chronic arthritis that may mimic rheumatoid arthritis. Haemophilus influenzae is a short, gram-negative rod that can cause osteomyelitis in children. Staphylococcus aureus is the most common cause of osteomyelitis, but the Gram stain would show gram-positive cocci. Treponema pallidum infection, also a sexually transmitted disease, can lead to syphilitic gummas in the tertiary phase of syphilis that may produce joint deformity. There is no preceding urethritis, however. Tertiary syphilis may be preceded years earlier by a primary syphilitic chancre.