A 33-year-old woman begins seeing a new boyfriend. Several weeks later, a week after her menstrual period, the woman develops a painful, swollen right knee. The most likely causative organism has which of the following characteristics?

The causative organism is Neisseria gonorrhoeae, which is a sexually transmitted gram-negative coccus that can live in phagocytic vacuoles in neutrophils and macrophages. The organism is the most common cause of septic ahritis in otherwise healthy, sexually active adults. Hyphae and spores would be features of Candida, which usually causes ahritis as a rare complication of systemic candidiasis in immunocompromised patients. The Chlamydia are intracellular organisms that cannot make ATP and do not usually cause septic ahritis. Mycoplasma do not have true cell walls and do not usually cause septic ahritis. Ref: Ray C.G., Ryan K.J. (2010). Chapter 30. Neisseria. In C.G. Ray, K.J. Ryan (Eds),Sherris Medical Microbiology, 5e.