What is the most likely causative organism in a young female with septic arthritis and purpuric skin lesions?

In a young female presenting with septic arthritis accompanied by purpuric skin lesions, the most likely causative organism is Neisseria gonorrhoeae. This is particularly relevant in sexually active young women, as gonococcal infection can lead to disseminated gonococcal infection (DGI), which not only manifests as septic arthritis but also presents with a characteristic skin rash that can appear purpuric.

DGI typically causes joint pain and swelling due to the infection spreading from the urogenital tract through the bloodstream. The purpuric skin lesions are indicative of vasculitis or other systemic effects of infection, which further supports the association with gonorrhea. While other bacteria can cause septic arthritis, the specific combination of a young female patient and these clinical findings is strongly suggestive of gonococcal infection.

Other organisms like Streptococcus and Staphylococcus, although they can also cause septic arthritis, do not typically present with the purpuric skin lesions seen in this scenario. Chlamydia, while also a sexually transmitted infection that can cause reactive arthritis, usually does not lead to the acute purpuric skin lesions nor is it as commonly associated with septic arthritis as gonorrhea. Therefore, gonorrhea is