Final answer:
The infectious cause of reactive arthritis confirmed by synovial fluid showing sterile inflammation is Chlamydia trachomatis. This type of arthritis follows infection elsewhere, not a direct joint infection. Option a.
Step-by-step explanation:
The infectious cause of reactive arthritis that is confirmed by synovial fluid analysis showing signs of sterile inflammation is A) Chlamydia trachomatis. When the synovial fluid is analyzed, no living bacteria are present, hence the term 'sterile inflammation'. This is due to the body's immunological response to a past infection rather than an ongoing infection within the joint itself. On the other hand, Streptococcus pyogenes, Escherichia coli, and Staphylococcus aureus are typical pathogens that directly infect the joint, which would be reflected in synovial fluid analysis with the presence of bacteria and a positive culture. Reactive arthritis usually follows infection elsewhere in the body, particularly urogenital or gastrointestinal infections, and C. trachomatis is a common preceding infection.