This file owns the monoarthritis differential as a clinical reasoning framework: synovial fluid analysis as the central diagnostic test (the 'tap the joint' decision and interpretation); crystal arthropathies (gout and pseudogout/CPPD) — pathogenesis, crystal morphology, clinical syndromes, and treatment principles; septic arthritis — when to tap, synovial fluid criteria, organism patterns by demographic, and management; Lyme arthritis as the post-infectious mimic; disseminated gonococcal infection (DGI) as the migratory-then-mono/oligo classic; and osteoarthritis as the mechanical baseline of joint pain.
Prerequisites
MSK Anatomy, Histology, Embryology & Joint Biology (synovial joint structure: type A macrophage-like and type B fibroblast-like synoviocytes; normal synovial fluid <200 WBC/μL with <25% PMNs, glucose ~serum, no crystals, sterile; articular hyaline cartilage avascularity and reliance on diffusion)Bone & Cartilage Biology, Mineral Homeostasis (subchondral bone biology and the cartilage-bone unit; chondrocyte biology)Approach to Inflammatory Arthritis & Rheumatoid Arthritis (the four-pillar inflammatory arthritis framework — applied here as the 'is this monoarthritis inflammatory or mechanical?' entry question; the inflammatory vs mechanical pain pattern)Vasculitides & Systemic Granulomatous Disease (the vessel-size + ANCA framework — vasculitis as a rare monoarthritis mimic when vessel involvement is articular)Type IV cell-mediated inflammation in crystal arthropathy NLRP3 inflammasome activation; neutrophil biology in septic arthritisBasic biochemistry (purine metabolism — hypoxanthine → xanthine → uric acid via xanthine oxidase; relevance to allopurinol/febuxostat)Basic microbiology (S. aureus, N. gonorrhoeae, B. burgdorferi, M. tuberculosis — organism biology owned in Microbiology)